Switch to a branded/customised version of sphinx-book-theme

[ptheywood]

Changes include:

• Customised colours, including backgrounds, links, code highlighting etc
  • Uses a darkened N8 Orange to achieve meet a11y guidelines for contrast
• Padding / margin customisations via CSS
• RHEL 8 migration announcement, which can be easily removed or adjusted in the future.
  • relies on JS to correctly set the link, when the absolute path is not known at the right time.
• Customised navigation search box with a submit button for a11y
• Customised footer template to remove unwanted automatic content
  • Adds acknowledgement of Sphinx and The executable book project.

Also switches to the default pygments syntax highlighting theme which is one of the few with good contrast for a11y

Closes #87 Closes #80 Part of #42

[ptheywood]

Hosted demo (temporary): https://ptheywood.uk/bede-docs-demo-book/