[MNT] Update PR description template and developer documentation to point to `main` instead of `develop`

[satvshr]

A small issue: the current template refers to develop as the primary branch, but our primary branch is main.

[revaarathore11]

@satvshr can i work on this?

[satvshr]

@revaarathore11 as far as I know this is something only the dev team can change, but if I am wrong, please go ahead as I am not working on it.

[Akarshkushwaha]

@PGijsbers @sahithyaravi @satvshr can i work on this issue ?