Use unique variable names in documentation and configuration

[agjohnson]

We are currently suggesting the following configuration:

# https://docs.readthedocs.io/page/reference/environment-variables.html
project = os.environ["READTHEDOCS_PROJECT"]
version = os.environ["READTHEDOCS_VERSION"]

# Include results from subprojects by default.
rtd_sphinx_search_default_filter = f"subprojects:{project}/{version}"

But these variables collide with the Sphinx configuration variables project and version, which aren't always the project/version slug -- they can be a verbose name established by the documentation author.

We should prefix our variables in documentation to make them unique, but it also looks like the default configuration uses project directly, which will have side effects if the project name is not the RTD slug.

https://github.com/readthedocs/readthedocs-sphinx-search/blob/main/docs/configuration.rst?plain=1#L25