Link from projection parameter to gallery

[maxrjones]

Description of proposed changes

This PR links to the projections gallery from the projection parameter in the API docs. I do not think the projections common text (see basemap for an example) provides enough information to be usable on its own, so this provides a link to more information.

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.

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[seisman]

One of the related doctests fails:

_______________ [doctest] pygmt.helpers.decorators.fmt_docstring _______________
179     ...
180     ...     Parameters
181     ...     ----------
182     ...     {R}
183     ...     {J}
184     ...
185     ...     {aliases}
186     ...     '''
187     ...     pass
188     >>> print(gmtinfo.__doc__)
Differences (unified diff with -expected +actual):
    @@ -11,5 +11,5 @@
         *Required if this is the first plot command*.
         *projcode*\[*projparams*/]\ *width*.
    -    Select map projection.
    +    Select map :doc:`projection </projections/index>`.
     <BLANKLINE>

You also need to update this line

pygmt/pygmt/helpers/decorators.py

Line 201 in b1d50d0

Select map projection.