Update maintainers guides

[seisman]

Preview: https://pygmt-dev--2916.org.readthedocs.build/en/2916/maintenance.html

[seisman]

@weiji14 Do you want to add a section like "Continuous Benchmark" to this guide?

[weiji14]

@weiji14 Do you want to add a section like "Continuous Benchmark" to this guide?

I did think about it when writing the docs in #2908, but wasn't sure what to include yet in terms of guidelines. Maybe after a few weeks/months when we've got some experience with benchmarking, then we can add some words in.

[seisman]

@weiji14 Do you want to add a section like "Continuous Benchmark" to this guide?

I did think about it when writing the docs in #2908, but wasn't sure what to include yet in terms of guidelines. Maybe after a few weeks/months when we've got some experience with benchmarking, then we can add some words in.

Sounds good.