feat(sitemap): add accurate lastmod from each doc's last content change

[amaan-bhati]

What has changed?

Adds an accurate <lastmod> to the docs sitemap, and fixes the "Last updated" date and dateModified on every doc page.

Before this PR:

• The sitemap had no <lastmod> (the sitemap plugin's lastmod option was unset).
• The deploy checkout was shallow (fetch-depth: 1), so every page reported the date of the newest commit. Live check: every page I sampled had the same dateModified (2026-10-11).

What this PR does:

• Content-based last-update date (src/lastUpdate/contentLastUpdate.js): for each doc, walks git history and uses the last commit that changed the Markdown body.
  • Skips front-matter-only and whitespace-only commits.
  • Follows copies and renames, so cutting a docs version (e.g. v3 to v4) does not reset every page's date.
  • Commits that change bodies without changing meaning can be listed in .lastmod-ignore-revs (works like .git-blame-ignore-revs).
  • A hand-written last_update.date in front matter still wins.
  • Injected through markdown.parseFrontMatter, so no doc files change.
• Sitemap: lastmod: "date" in the sitemap plugin config.
• CI: fetch-depth: 0 in main.yml (deploy) and build_and_check.yml (PR preview).
• Safe fallback: in dev, with DOCUSAURUS_DISABLE_LAST_UPDATE, or in a shallow clone, it logs a warning and falls back to Docusaurus's default behaviour.

Why: Google and Bing use <lastmod> only when it is consistently accurate. Dates that move without real changes get ignored for the whole sitemap.

Type of change

• New feature (non-breaking change which adds functionality).
• Bug fix (non-breaking change which fixes an issue).

How Has This Been Tested?

• yarn build passes.
• build/docs/sitemap.xml: 216 of 217 URLs now carry <lastmod> (only /docs/search/ has none), with dates from 2023-10 to 2026-10 instead of a single date.
• Pages copied in the v4 cut show their real v3-era dates, not the cut date (Jan 2026). Example: concepts/general-glossary has dateModified 2023-11-29; running-keploy/cli-commands has 2026-10-07.
• Spot-checked the most common resulting commits (docs: one Keploy across the site - no editions, accurate native macOS/Windows, eBPF on Linux #918, feat(docs): site health + AI-citation fixes (A1, A2, A8 …) #897): both are real content changes, so no ignore entries are needed yet.

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.