Moving process docs for feature flags to handbook

What does this MR do?

• the feature flag working group is consolidating and clarifying the existing feature flag documentation. See #249129 (closed) (comment 523228764) for more details. This commit removes the process documentation and gitlab-com/www-gitlab-com!76630 (merged) moves it to the handbook.

The plan is, in MRs:

1. Docs site MR !55891 (merged)
   1. process.md content moved to handbook, and becomes a redirect to the lifecycle handbook page
   2. index.md contents moved to handbook, and content replaced by development.md content
2. Handbook MR number 1 gitlab-com/www-gitlab-com!76630 (merged)
   1. process.md content copy/pasted to handbook page, all links fixed
   2. index.md content copy/pasted to handbook page, all links fixed
3. gitlab-docs MR gitlab-docs!1582 (merged)
   1. To remove the links in the sidebar of the docs site

Related issues

#249129 (comment 523228764)

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#move-or-rename-a-page

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• [-] Update the link in features.yml (if applicable)
• Assign one of the technical writers for review.

/cc @darbyfrey