Build the Memory Management Reference from the MPS tree

[rptb1]

Changes to the MPS manual could break the Memory Management Reference. Incorporating the build will allow us to check it, both manually and as part of CI.

See #166 (comment) .

See also #165 though this branch is not intended to resolve that issue, only mitigate risks.

[rptb1]

Here is a partial build of the Memory Management Reference using Read the Docs from this branch https://mmref.readthedocs.io/en/branch-2023-03-03-make-mmref/

This is achieved by setting the MMREF environment variable to 1 in the mmref Read the Docs configuration.

It's only a partial build because we post-process the Sphinx output to produce https://www.memorymanagement.org/ and Read the Docs is not running that script. It probably could.

[rptb1]

I am mystified by the errors from the Read the Docs builds. I have submitted readthedocs/readthedocs.org#10103 .