Reference an OpenSearch description file that sits outside the content
of any specific version so we only need to maintain one and so that
it can reference the latest version.
This was previously added in a custom branch for building the
cmake.org reference documentation.
Leave the option undocumented since it is not for general use. We can
simply pass it through `SPHINX_FLAGS` when building for publication on
`cmake.org`.
The latter is now the preferred URL for visiting cmake.org with a
browser. Convert using the shell code:
git ls-files -z | xargs -0 sed -i 's|http://www\.cmake|https://cmake|g'
Brad King