[RTL] Warn if RTL CSS not found + Hugo mod fix + more

[chalin]

• Fixes Ensure RTL works when Docsy is used via Hugo modules #2025
• ci:prepare now copies over a cached copy of the Bootstrap RTL CSS
• Adjusts the head partial:
  • Warn if Bootstrap RTL CSS is needed for some pages but not found; and embeds the warning as a <meta name="docsy-info" element of affected HTML pages
  • Report an error if the main CSS file isn't found
• Adds a mount point for Hugo-module-based projects so that the BS RTL file
• CONTRIBUTING.md typo fix

[chalin]

Merging now, but feedback is still welcome @LisaFC @geriom @emckean @fekete-robert @deining. I'll be testing this soon on the docsy-example.