jekyll(last_modified_at): fix for remote resources and data files

[crazy-max]

follow-up #15994 (comment)

Fix last_modified_at plugin that overrides last_modified_at frontmatter value for remote resources. And use data files for commands reference instead of stub files to compute the correct last modification date.

Also adds DOCS_ENFORCE_GIT_LOG_HISTORY option to enforce git log history for local dev for testing purpose.

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[crazy-max]

@usha-mandya @dvdksn Not related to this PR but link to Hub pricing is broken:

- ./_site/subscription/faq/index.html
  *  https://
hub.docker.com/billing/plan/update is an invalid URL (line 255)
     <a href="https://
hub.docker.com/billing/plan/update" target="blank" rel="noopener" class="">the billing</a>
  *  linking to internal hash #pricing-plans that does not exist (line 210)
     <a href="/subscription/#pricing-plans" target="blank" rel="noopener" class="">billing</a>

Fixed it, see last commit

[dvdksn]

LGMT, ptal @jerae-duffin