Use substitutions to show the minimum required Python and GMT versions dynamically in installation guides

[seisman]

Description of proposed changes

Use sphinx's substitutions feature (https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#substitutions) to insert the minimum required Python and GMT versions. The version strings are obtained from the PyGMT package dynamically so that we don't have to manually update the doc/install.rst file.

Preview: https://pygmt-dev--2488.org.readthedocs.build/en/2488/install.html

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 commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

Ping @GenericMappingTools/pygmt-maintainers for one more reviewer for this PR.