Team Gallery page: Restructure in three subsections

[yvonnefroehlich]

Description of proposed changes

This PR updates the Team Gallery page:

• Change title to PyGMT Team
• Strucutre involved people in the three subsections "Founders", "Active Maintainers", "Distinguished Contributors"
• Add policy reagarding removing a maintainer - where?
• Add reminder in the Release checklist issue template

Preview: https://pygmt-dev--3240.org.readthedocs.build/en/3240/team.html

Fixes #1212

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash command is:

• /format: automatically format and lint the code

[seisman]

Looks good to me.