Update links in docs, readme, metadata

[simonbowly]

Before project is public, page links need to be updated in:

• [x] pyproject.toml (appears on pypi page)
  • [x] docs link needs to change from /beta to /stable
• [x] readme (appears on github page)
• [x] index.rst page (front page of readthedocs)

Specifically:

• [x] https://support.gurobi.com/hc/en-us/community/topics for support/questions/discussion
• [x] https://github.com/Gurobi/gurobipy-pandas/issues for bugs/feature requests
• [x] https://support.gurobi.com for Gurobi or gurobipy bugs

[simonbowly]

Links updated in readme, docs and metadata. Pushed 0.2.1b0 to pypi to update the page there.