docs: move away from readthedocs

[jecluis]

On Nov 15 2023, I got this email from readthedocs:

Previously, manually configured webhooks from integrations did not have a secret attached to them.
In order to improve security, we have deployed an update so that all new integrations will be created
with a secret, and we are deprecating old integrations without a secret. You must migrate your
integration by January 31, 2024, when they will stop working without a secret.

We are contacting you because you have at least one integration that does not have a secret set.
These integrations are:

   [snip]

If you aren't using an integration, you can delete it. Otherwise, we recommend clicking on
"Resync webhook" to generate a new secret, and then update the secret in your provider's settings as
well. You can check [our documentation](https://docs.readthedocs.io/en/stable/guides/setup/git-repo-manual.html)
for more information on how to do this.

You can read more information about this in our blog post:
https://blog.readthedocs.com/security-update-on-incoming-webhooks/.

We have a couple of months to deal with this, but this should be done as soon as possible.

[jecluis]

We should focus instead on having our documentation being built and provided through the website, which relies on docusaurus -- which should be able to do this fairly easily.