Defined the remaining missing cpr_string_*() functions.
Modernized the project's coding conventions.
Added some notes on the C++-compatible object layout.
Defined a custom theme based on sphinx_rtd_theme.

Also, disabled all custom Sphinx extensions for the time being.
Customized the Sphinx theme to enable the flyout and GitHub links.
Removed the custom theme in preparation for upstream subtree merge.
Squashed 'doc/manual/.themes/' content from commit 8ed3c9e

git-subtree-dir: doc/manual/.themes
git-subtree-split: 8ed3c9e41752d40bd37bf9182dc76f3951ee4d77
Merged the https://github.com/dryproject/sphinx-themes subtree.