Update Readthedocs config to use Readthedocs Add-ons, per their migration/deprecation plan

[mperrin]

RTD is changing some of their infrastructure and it sounds like we need to update some conf files for consistency.

See https://about.readthedocs.com/blog/2024/07/addons-by-default/

Deadline Oct 7 2024.

[mperrin]

@BradleySappington FYI, in case you hadn't seen this issue I filed a while ago. I expect this same fix is going to be needed in a whole bunch of our repos, potentially.

[BradleySappington]

@mperrin Thank you! I missed this

[BradleySappington]

@mperrin, I just turned on Add-ons for poppy and webbpsf via the RTD Dashboard and rebuilt a hidden test repo, all seems to work fine without any config file updates. Planning to leave them as is, and if there is a build error with the next release for any reason then I'll update accordingly.

[mperrin]

Oh excellent; I'm glad to hear it's that simple without any extra steps. Great.