Mainframe: Deprecate bootstrap-table + Basic 404 Page

[TasoOneAsia]

Proposed changes

This PR:

• Deprecates the <bootstrap-table> shortcode and implements the <table> shortcode as the Mainframe table.
  • Under the hood,bootstrap-table calls table with a default variant of "narrow" to maintain backwards compatibility
  • Bootstrap striped table handling not implemented.
  • Documentation needed
• Implements a simple 404 page with new grid handling.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[github-actions]

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[TasoOneAsia]

Narrow + wide table variants can be found on the example site here

[lamATnginx]

LGTM. Runs fine locally but the preview does not use the right 404 page even when cache cleared.

[nginx-jack]

LGTM. Runs fine locally but the preview does not use the right 404 page even when cache cleared.

That's an oddity with how azure handles 404's unfortunately. Will be fine in dev/staging/prod.