Docs: updates docs development guidelines

What does this MR do?

• Follow-up from https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/108
• Updates docs dev guidelines for Kramdown
• Reorder docs dev index for better flow (leaving all the content about previewing, testing, and linting to the end of the doc)
• Adds docs-only punctuation rules (closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53115)
• Allow inline links only (closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53160)

For reviewers

There are some less relevant changes in this MR. The most significant ones are:

• Punctuation rules: https://gitlab.com/gitlab-org/gitlab-ce/blob/27f0947832b327dd329862cb047132618943276a/doc/development/documentation/styleguide.md#L80
• List items rules: https://gitlab.com/gitlab-org/gitlab-ce/blob/27f0947832b327dd329862cb047132618943276a/doc/development/documentation/styleguide.md#L95
• Examples for lists (we could use better examples, I think): https://gitlab.com/gitlab-org/gitlab-ce/blob/27f0947832b327dd329862cb047132618943276a/doc/development/documentation/styleguide.md#L122

Review app

• Main index: http://review-docs-docs-5902aq.178.62.207.141.xip.io/ce/development/documentation/index.html
• Style guide: http://review-docs-docs-5902aq.178.62.207.141.xip.io/ce/development/documentation/styleguide.html

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE)

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8363