doc: Update the documentation for the monorepo benchmark

[punchagan]

This PR adds documentation for Dune developers to work with the monorepo benchmarks. The documentation explains how to run the benchmarks locally, view and analyze the results of the continously run benchmarks, and how to add new benchmarks.

[snowleopard]

Many thanks @christinerose! Agreed about "one-shot benchmarks".

[punchagan]

@christinerose Steve has added a new Maintenance section. Would you be able to review that? (The rest of the changes were already reviewed, and your changes/comments were incorporated)

[christinerose]

@christinerose Steve has added a new Maintenance section. Would you be able to review that? (The rest of the changes were already reviewed, and your changes/comments were incorporated)

Done! Thanks! I'm always happy to help.