Add a rake docs:preview task serving the docs site locally

## Motivation and Context

The documentation site under `docs/` had no development command: previewing a change meant hand-assembling
a Jekyll environment, since the site only builds for real when a release publishes it through
`bin/generate-gh-pages.sh` and the GitHub Pages runtime. `rake docs:preview` now builds and serves
the site at http://localhost:4000 (override with `PORT`), with watch-based rebuilds on edit.

The preview dependencies live in their own `docs/Gemfile` rather than the gem's Gemfile:
`github-pages` mirrors the runtime that builds the released site (jekyll-remote-theme,
jekyll-redirect-from, and the Jekyll version Pages actually runs), which keeps the preview faithful
but is far too heavy to impose on the gem itself. The task subshells into that bundle with
`Bundler.with_unbundled_env`, the same delegation shape the conformance tasks use for npx.
Two support pieces keep the pinned toolchain running on current Ruby. `docs/_preview/taint_shim.rb` restores
the taint API as no-ops: Liquid 4.0.3 calls `obj.tainted?` on every variable render,
ahead of its own lax-mode check (`liquid/variable.rb:124`), so on Ruby 3.2 and later,
where the taint API was removed, the first layout render dies with NoMethodError, while on Ruby 3.1
and earlier the methods still exist as deprecated no-ops and the shim is inert. The shim is loaded only into
the preview process via RUBYOPT. The former default gems the pinned Jekyll expects are listed in the docs Gemfile.
The task also regenerates `docs/_data/versions.yml` from the version tags, mirroring what `bin/generate-gh-pages.sh`
produces at deploy time, so the nav footer shows the released-gem version line exactly as it does on the published site.
Build products and the generated data file are ignored.

## How Has This Been Tested?

By running the task on a spare port and verifying the served pages, the footer's version line,
and that a SIGINT stops it cleanly with exit 0; `git status` stays clean while it runs (all build products ignored).
Verified on both Ruby 3.1.5 (shim inert) and Ruby 4.0.6, where a shimless build reproduces the NoMethodError
and the task runs green with the shim. RuboCop and the full suite are green.

## Breaking Changes

None. The task and its Gemfile are development-only additions; the gem and its dependencies are untouched.

docs/Gemfile

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# Dependencies for the local docs preview (`rake docs:preview`), kept out of the gem's own Gemfile:
+# github-pages mirrors the GitHub Pages runtime that builds the released site
+# (jekyll-remote-theme, jekyll-redirect-from, and the Jekyll version Pages actually runs).
+source "https://rubygems.org"
+
+gem "github-pages", group: :jekyll_plugins
+gem "webrick"
+
+# Former default gems that the Jekyll version pinned by github-pages still requires on Ruby 4.0.
+gem "base64"
+gem "bigdecimal"
+gem "csv"
+gem "logger"

docs/_preview/taint_shim.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# Liquid 4.0.3 (pinned by github-pages) still calls the taint API that Ruby 3.2 removed.
+# Restore it as a no-op for the local docs preview only; `rake docs:preview` loads
+# this file via `RUBYOPT`, so nothing outside the preview process is affected.
+class Object
+  def tainted?
+    false
+  end
+
+  def taint
+    self
+  end
+
+  def untaint
+    self
+  end
+end