chore(docs): enable gitlab_docs Vale styles for docs/source

Description

docs/.vale/gitlab_docs/ is now listed in BasedOnStyles.

Two gitlab_docs rules are switched off for this project:

• RelativeLinks: docs/source is generated from the Go help text in internal/commands, and the same strings print in the terminal for glab <command> --help, where a relative Markdown path means nothing. Full URLs to docs.gitlab.com are correct here.
• InternalLinksCode: The rule's pattern matches links into a config/ directory, one of the top-level directories in gitlab-org/gitlab. glab has its own config commands, so every generated link to a config subcommand page is a false positive.

Verification

vale --minAlertLevel error docs, the command check_docs_markdown runs, reports 0 errors across 295 files. Before the two exclusions it reported 20 errors: 17 RelativeLinks and 3 InternalLinksCode. Checked locally with Vale 3.12.0; CI uses 3.18.0.

Related issues and merge requests

• Slack thread