feat: generate GitLab Docs navigation block for glab

What

Makes the GitLab CLI (glab) navigation submenu in docs-gitlab-com's data/en-us/navigation.yaml generated from the glab cobra command tree instead of hand-maintained.

Generation lives in the existing cmd/gen-docs binary: alongside the web docs, make gen-docs now also walks the command tree and writes the nav submenu to a checked-in artifact, docs/navigation-glab.yaml. The GitLab Docs site (docs-gitlab-com) splices this block into navigation.yaml.

• cmd/gen-docs/docs.go gains a walk()/genNav() step and a --nav-path flag (default ./docs/navigation-glab.yaml); the nav is emitted for web docs only, not man pages.
• make gen-docs now generates both the docs pages and the nav artifact — no separate target or binary.
• The existing pre-commit (check-docs) and pre-push (check-generated) lefthook checks, plus the check_docs_update CI job, already run make gen-docs and diff docs/, so they now guard the nav artifact too — it can't silently drift from the command definitions, locally or in CI.
• For each available command it emits title = the command path (e.g. glab ci cancel job) and url = the docs path (e.g. cli/ci/cancel/job/), with a submenu for commands that have subcommands. Help, hidden, and deprecated commands are skipped, matching the generated docs pages.

Why

Today the glab entries in docs-gitlab-com's navigation.yaml are hand-maintained and drift from the command tree (missing glab prefixes, title typos, ordering). They're fully derivable from cobra, so this makes them generated and guarded instead.

Tracking

Tracked in gitlab-org/technical-writing/docs-gitlab-com#671 (POC), part of the broader effort in gitlab-org/technical-writing/docs-gitlab-com#669.

Companion MR

This is the generator half. The consuming half (sentinels + splicer + synced navigation.yaml) is a companion MR in docs-gitlab-com (branch 669-glab-nav-poc). Recommended merge order: this MR first, so the artifact exists on main for the docs side to consume.

Test

make gen-docs                                  # regenerate docs + nav
git diff --exit-code docs/navigation-glab.yaml # exit 0 == in sync with the command tree

The pre-push check-generated hook and the check_docs_update CI job exercise the same guard automatically.

Scope note

This is V1 (generator + drift guard). Cross-repo MR automation is intentionally out of scope — see the Stage 3 design sketch on gitlab-org/technical-writing/docs-gitlab-com#669.