refactor(Contentful): Standardize the use of the 'slug' field on Contentful

What is changing in this MR?

• Remove the trailing and leading slashes from the slug property across the Page content type pages to have a proper standardization of how to use this property.

Production	Review app	Entry
https://about.gitlab.com/pricing/feature-comparison	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/pricing/feature-comparison	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/3eqG2bxweO8ao6bKDOIaXw
https://about.gitlab.com/customers/all	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/customers/all	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/J3iBHyAlk60tKlNq15VSJ
https://about.gitlab.com/events	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/events	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/3ooZbNzGphncsmMpi73gWV
https://about.gitlab.com/install/ce-or-ee	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/install/ce-or-ee	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/36I2vbruRpa7ED35IlaeTd
https://about.gitlab.com/press/press-kit	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/press/press-kit	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/6GOTryYCykCvXnrl8KIKWN
https://about.gitlab.com/install/security	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/security	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/FdoqitY5josrv7KZJMGlf
https://about.gitlab.com/stages-devops-lifecycle	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/2zDkipFCalG4w9xEUFKrrP
https://about.gitlab.com/stages-devops-lifecycle/configure	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/configure	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/laGsOSNJ0bN0QuOpBwGxB
https://about.gitlab.com/stages-devops-lifecycle/continuous-delivery	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/continuous-delivery	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/25I71IzdR3xlF5YXOgyiMd
https://about.gitlab.com/stages-devops-lifecycle/create	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/create	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/XXKb3vQw5oSgVUoAHWRih
https://about.gitlab.com/stages-devops-lifecycle/enablement	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/enablement	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/Kyu3UTVTIkTmbHKIccMK1
https://about.gitlab.com/stages-devops-lifecycle/govern	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/govern	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/5HJmSJkZ0Ggce4pW5Qn1th
https://about.gitlab.com/stages-devops-lifecycle/monitor	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/monitor	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/4TApz4T9yWot0hpx5kZhFb
https://about.gitlab.com/stages-devops-lifecycle/package	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/package	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/5v7ZltSjjVZRS0538Jkvjt
https://about.gitlab.com/stages-devops-lifecycle/plan	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/plan	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/7snT9mAq6pD1vIILWaUcU4
https://about.gitlab.com/stages-devops-lifecycle/release	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/release	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/40NOyX2iXWzn2xRN0z5Kop
https://about.gitlab.com/stages-devops-lifecycle/secure	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/secure	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/3Y2XJiH60Bsa1OQJjh5A57
https://about.gitlab.com/stages-devops-lifecycle/verify	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/stages-devops-lifecycle/verify	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/2spEijwtJcmFXvhYD95zpI
https://about.gitlab.com/terms	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/terms	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/1oPdJwLKUDokfvAz1rqxBT
https://about.gitlab.com/value-stream-workshop	https://3367-eng-add-proper-format-validations-to-the-slug-field-in-our.about.gitlab-review.app/value-stream-workshop	https://app.contentful.com/spaces/xz1dnu24egyd/environments/master/entries/5nM8kmQAhJbHnuCZDUolMy

Steps to deliver

1. Merge this MR
2. Just right after the previous step, publish this Contentful release
3. Add the ^(?!\/$)(?!\/)(?!.*\/$).*$|^\/$ regex to the slug field in the Page content type

Build Variables:

• Use Contentful Preview API

Closes #3367 (closed)