Only build documentation if files under docs folder changed

[r0qs]

This PR addresses one suggestion from: #16125.

I've added a dynamic configuration in our CircleCI setup to run documentation builds only when files in the docs folder are modified (reference: https://circleci.com/docs/dynamic-config/#use-dynamic-configuration).

Note that workflow reruns may fail due to how CircleCI's dynamic setups work:

A pipeline can only be continued once. A pipeline cannot be continued with another setup configuration.
Workflow reruns will fail as part of this restriction. Rather than rerun a setup workflow you can trigger a new pipeline.

To be able to do that, I enabled the following option in the project CircleCI configuration: Project settings > Advanced -> Dynamic config using setup workflows

[r0qs]

Apparently, we cannot use the same config for different logics, so we will need to split the configuration in 3 to be able to perform such separation of workflows. Basically, we will have a setup configuration in config.yml and more two, one for code changes and one for docs. We may add more later. See: https://circleci.com/docs/using-dynamic-configuration/#generate-a-config-file-based-on-modified-files

[sbillig]

I think this should be run as a github action and avoid a circleci job at all, to avoid the $15 charge each time a new person submits a trivial docs change.