Make documentation build reproducible

[klmurussi]

What's the problem this feature will solve?

Currently, the documentation build for pytest might not be fully reproducible across different build environments. This means the generated documentation output could vary depending on environmental factors, such as the presence or absence of Read the Docs (RTD) environment variables (READTHEDOCS and READTHEDOCS_VERSION_TYPE).

Describe the solution you'd like

Forcing the IS_RELEASE_ON_RTD ( in doc/en/conf.py file) variable to True during the documentation build.

Additional context

This proposal is based on an existing and active patch within the Debian packaging for pytest.

This proposal is based on an existing and active patch within the Debian packaging for pytest. It's not ideal for Debian to carry patches that diverge from the upstream project.