Write blog post about `build.commands`

[humitos]

We have done good work on the documentation page about build.jobs and build.commands, https://docs.readthedocs.io/en/latest/build-customization.html. However, we haven't write a blog post announcing build.commands in beta state.

I think it's a good timing for this since we have been testing it during some months and it's a good feature as it is, even with the current limitation that it has.

We wrote a blog post about build.jobs that we can use as a reference: https://blog.readthedocs.com/user-defined-build-jobs/

[ericholscher]

Strong agree on this! There's lots of Use $x with Read the Docs content that should follow this announcement as well. Either as a blog, or guide, or both!