update guide to include how to sync repo

[reshamas]

https://docs.pymc.io/en/latest/contributing/pr_tutorial.html

reminder: to use different branch

$ git fetch upstream
$ git rebase upstream/main

delete a branch
https://stackoverflow.com/questions/2003505/how-do-i-delete-a-git-branch-locally-and-remotely

[reshamas]

@OriolAbril

[OriolAbril]

to give some more context, the current page has all the steps for creating the first ever PR, it doesn't help much for the 2nd by which time users are still not familiar with git.

It might also be worth to get information from https://pymc-data-umbrella.xyz/en/latest/sprint/docstring_tutorial.html