Reconcile environment for development and docs building

[HangenYuu]

Description

Current issue

The current environment.yml is not fully supported for building docs locally. The documentation for new contributor also does not mention this file yet.

Proposals

• Reconcile the file above with the file used for building doc in the workflow. Adding git+https://github.com/pymc-devs/pymc-sphinx-theme in the file is enough, I have tested locally.
• Update the Developer Start Guide to use environment.yml instead.