Use pyproject.toml to build docs on RTD

[stevepiercy]

We currently use a separate file to build docs on Read the Docs.

sphinxcontrib-httpexample/.readthedocs.yaml

Line 35 in 07788d4

- requirements: docs/requirements.txt

We should instead use pyproject.toml to unify installation.

• Update .readthedocs.yaml
• Delete docs/requirements.txt
• Verify that all doc builds, both of versions and pull requests, and tests continue to function.

See also #51, as implementation of this issue would likely close that one.