Update release process and checklist template

[weiji14]

Description of proposed changes

Simplify our release checklist template further, following up on suggestion from #587 (comment). See also #444 and #446. Changes made are:

• Remove git tag step, since making a Github release will auto create one
• Add checklist item to bump PyGMT version on try-gmt
• Split announcement sites (Forum/Website/ResearchGate) into separate checklist items

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.