Fix compilation of deprecations doc, and test it in CI

What does this MR do?

This improves the autogenerated deprecations doc:

• The doc was not compiling when there were features added, so thanks to a suggestion from @godfat-gitlab, this is fixed.
• Adds a new raketask that compiles a copy of the the doc, and verifies that it matches the current doc.
• Adds a CI job to run the new raketask, which will ensure the doc never gets out of sync. It'll fail if you try to submit an MR without compiling the doc. The rules for the job ensure it only runs when these files are edited:
  • Feature YAML files
  • The deprecations doc
  • The deprecations doc .erb template
  • The files related to the rake task itself.
• The doc was located in a new top-level directory (doc/deprecations), which we'd like to avoid (https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#folder-structure-overview). This moves the doc under doc/update, which is a related topic.
• Copy edits the text.
• Adjusts the spacing in the template to resolve linting errors when generating the doc with features listed:

Before	After

• Related to !66770 (merged)
• Related to gitlab-com/Product#2864 (closed)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team