Add option to only aggregate documentation from selected package versions

[kdayday]

Summary

Adds optional control over which version directories are copied into each aggregated doc tree, and wires a “See All Versions” entry into Documenter’s version <select> that opens the upstream package site (GitHub Pages by default, or a custom URL) in a new tab. This helps when aggregating many repos with long gh-pages histories so the combined site stays closer to GitHub Pages limits (e.g. the 1 GB cap).

New MultiDocRef options

• include_versions::Union{Vector{String}, Nothing} — When non-nothing and non-empty (e.g. ["stable", "dev"]), only those top-level version entries are copied from upstream. All root-level files (not only index.html / versions.js) are copied; .git is skipped. Version dirs are copied with follow_symlinks = true so symlinks such as stable → v1.0.0 become real directories in the aggregate.

• all_versions_url::Union{String, Nothing} — Absolute http(s) URL used for “See All Versions” when include_versions is in use. If unset and giturl is a github.com clone URL, the URL is derived (e.g. https://github.com/org/Pkg.jl.git → https://org.github.io/Pkg.jl/). Non-GitHub hosts are not auto-derived; set all_versions_url explicitly. If neither yields a usable URL, version limiting still runs, but the extra selector option is not injected.

Behavior when include_versions is set

• After the selective copy, the aggregate root index.html is regenerated (not taken from upstream) so it meta-refreshes to first(include_versions), avoiding stale upstream redirects (e.g. old org URLs).

• versions.js is rewritten so DOC_VERSIONS lists only the kept versions.

• Each processed doc HTML gets a small JSON config <script> (before </body>) consumed by assets/default/see_all_versions.js. That script appends the sentinel option, opens the target URL in a new tab on choose, and restores the dropdown. It also replaces a legacy inline “see all versions” script if present.

• see_all_versions.js is registered as a global default script (so it ships on every build), but it only does work when the config block exists (i.e. when injection ran for that ref).

When include_versions is unset

Behavior matches main: full copy of upstream, no versions.js rewrite, no “See All Versions” injection for this feature path.

Other changes (same branch)

• PageFind / NodeJS JLL: Harden npx / npm invocation (executable path resolution and working directory) for Windows and newer Julia where CI previously failed to spawn pagefind.

[kdayday]

@pfitzseb Hello, is it possible to get a review on this PR? I believe the remaining Windows errors are the same as currently exist on main. Thanks so much!