Renames "Manage" stage to "Foundations" in docs metadata

What does this MR do?

In gitlab-com/www-gitlab-com!134359 (merged), the handbook is being updated to rename the Manage stage to Foundations. We'll need to update the docs metadata as well. We'll need a second MR with a longer pipeline to update our backend tooling, but I'll place that in a second merge request. (!162338 (merged))

I'm marking this MR as dependent on gitlab-com/www-gitlab-com!134359 (merged) so it doesn't merge too soon.

Related issues

• Related to Rename "Manage" stage to "Foundations" (gitlab-com/www-gitlab-com!134359 - merged)
• Related to https://gitlab.com/gitlab-com/Product/-/issues/13078+
• Related to Renames "Manage" stage to "Foundations" in docs... (!162338 - merged) which updates the TW backend tooling separately.
• Related to TW stage and group change tasks for Manage -> F... (technical-writing#1033)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.