Docs: cover data lifecycle for build process

[agjohnson]

The build process is described technically here: https://docs.readthedocs.io/en/stable/builds.html

But this page is missing information on the lifecycle of data stored. We might want to communicate:

• Each commit, we clone the project's repository for Git operations, and then the files are removed
• Each build, we clone the project's repository, build documentation, and then remove files are the build process completes
• Build instances are single tenant, there are no shared resources between two builds
• We store build documentation until versions are deleted
• And perhaps it's worth noting that build artifacts in Sphinx include the source reST files (/_sources/*)