DOC: migrate documents to readthedocs

[wkliao]

Readthedocs has a nice integration with Github CI. A commit pushed to a branch
of Darshan's repo can trigger a rebuild of all Darshan's documents, which include

• darshan-runtime
• darshan-util
• darshan-modularization
• PyDarshan

A short developer guide is provided in file ./readthedocs/DEVELOPER_NOTES.md

[carns]

this is great! I'll leave it to you @wkliao to determine how to coordinate the timing (I don't know if we should merge this, add some tags in the format rtd expects, or update the web site)

[wkliao]

Because there is no new commits pushed to the main branch between 3.4.7 and this PR, we can simply merge this PR now.

If you would like to make a public announcement for the readthedocs migration soon, we can create a new tag now using the readthedocs required version format, e.g. 3.4.7.1, or v3.4.7.1. Such that readthedocs can identify it as a stable version.

Public announcement can be done in 2 places: GitHub repo README.md and Darshan web home page. The former can be done by a new PR.

If these sound fine with you, I will go ahead merge this PR.

[carns]

Sounds good, with the only suggested tweak being that we update the links on the Darshan web site to refer to readthedocs before we announce the migration.

Post-migration we need to:

• remove the old static documentation files that are hosted from CELS
• update the Darshan release checklist

[carns]

(so please do go ahead and merge and make tags whenever you are ready @wkliao )

[wkliao]

I pushed another commit about

update the Darshan release checklist

Updating URLs on CELS will be separated from this PR.

[wkliao]

A new tag, 3.4.7.1, has been created, which triggered readthedocs to recognize it as the latest stable release.
We can use this URL below as Darshan's official documents to update Darshan's home page.
https://darshan.readthedocs.io/en/stable/