[PATCH 0 of 3] nginx.org usability enhancements

[Liam Crilly]

Hello!

This is the first of a multiple patch series for nginx.org.

This changeset brings numerous usability enhancements while retaining the
familiar look and feel of the site.

The primary visual changes are:

*) Readability on mobile devices
   - The right-hand menu is not shown for narrow viewports. In this case a
     conventional 'hamburger' icon on the banner is used to show the menu.
   - Font sizes are more consistent, moving primarily to the rem metric.

*) Increased contrast and emphasis for code and configuration styles in the
   'pre' and 'code' blocks.

*) Dark mode honors the client's system preferences and is automatically
   applied.

The embedded stylesheet has been extracted into separate files to aid
ongoing development and maintenance. The special character layouts for the
redundant Hebrew and Chinese translations are preserved but have not
received further enhancements. The Russian and English stylesheets are
currently identical and will be consolidated in a future changeset. For the
timebeing, any changes to one must be copied to the other.

This changeset should apply cleanly without requiring configuration changes
to the underlying web server.

Cheers,
Liam.