Backport 19.5 docs pages missing from the stable branch

What does this MR do and why?

Backports three docs pages to 19-5-stable-ee. They merged to master after the stable branch was cut, but the 19.5 docs navigation links to them, so the 19.5 docs pipeline fails test:check_global_nav_entries.

Page	Original commit
doc/user/ai-governance/externally-connected-agents.md, plus the link in ai-governance/_index.md	bfe875612311, b8acef3e3b6b
doc/administration/dedicated/customer_api.md, plus tenant_overview.md	55307bf0cc36
doc/development/documentation/styleguide/ui_text.md, plus styleguide/_index.md, word_list.md, topic_types/task.md	53e910e4913f (docs files only)

Each file matches master. No other commits touched these files after the branch point.

What's left out

From 53e910e4913f, this MR leaves out the check_ui_text.rb lint script, the hugo_transformer.rb change, and the jenkins.md edit. The page uses the ui shortcode in its examples. The docs site renders it, but in-app help on 19.5 shows it as raw text.

Failing pipeline: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/pipelines/2951433813

Related issue: docs.gitlab.com release 19.5 (October, 2026) (gitlab-org/technical-writing/docs-gitlab-com#753)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.