Feat/gen toc

[VelmiraS]

What this PR does / why we need it:

Adds a new gen-toc subcommand that generates a navigation YAML file from a Docforge manifest. The generated structure reflects the dir / file / fileTree hierarchy defined in the manifest and can be used as input for SAP portal (toc.yaml)

Also fixes a non-deterministic ordering bug in Tree() output caused by Go map iteration randomness — without this fix, repeated runs of gen-toc on the same manifest could produce different orderings.

Which issue(s) this PR fixes:
Fixes ##471

Special notes for your reviewer:

The gen-toc subcommand does not download files — it only resolves the manifest and derives the navigation structure. The --strip-root flag removes the top-level directory prefix from all paths in the output.

The ordering fix in pkg/registry/repositoryhost/github_http_cache.go (sort.Strings(out) in Tree()) is a minimal, safe change — GitHub API already returns entries alphabetically, so the sort produces the same result as preserving API order.

Release note:

Added gen-toc subcommand to generate a navigation YAML from a Docforge manifest without downloading any files. Supports VitePress, MkDocs, SAP portal (toc.yaml), and other navigation formats. Use --strip-root to remove the top-level directory prefix from all paths.

[gardener-prow]

Adding label do-not-merge/contains-merge-commits because PR contains merge commits, which are not allowed in this repository.
Use git rebase to reapply your commits on top of the target branch. Detailed instructions for doing so can be found here.

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[gardener-prow]

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