List deprecated load_* functions in a separate table

[seisman]

Description of proposed changes

Address comment #2125 (comment)

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.

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

[willschlitzer]

Looks good.

Only issue is that there is a horizontal slider to read all of the description of the deprecated dataset functions. I don't think it's that big of a deal, and will be gone anyway in a few releases.

[weiji14]

Two minor suggestions, otherwise ok!