Convert projection table from reStructuredText to Markdown

[yvonnefroehlich]

Description of proposed changes

This PR tests / aims the conversion of the projection table from reStructuredText to Markdown (#3346 (comment), #3356 (comment)):

• Substitution definitions
• Sub / super scripts
• URL links within PyGMT documentation
• Readability: Line length, white spaces (and exception)
• | exception

Preview:

• reStructuredText (old): https://www.pygmt.org/dev/techref/projections.html
• Markdown (new): https://pygmt-dev--3363.org.readthedocs.build/en/3363/techref/projections.html

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 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

[yvonnefroehlich]

/format