Add common aliases xshift (X) and yshift (Y)

[weiji14]

Description of proposed changes

Used for shifting plots along the x and y dimensions. See https://docs.generic-mapping-tools.org/6.1/gmt.html#xy-full.

Live documentation preview for this PR/branch is at https://pygmt-git-common-aliases-xy.gmt.vercel.app/api/generated/pygmt.Figure.plot.html

Note that the xshift/yshift names follow upstream GMT long option name conventions at https://github.com/GenericMappingTools/gmt/blob/88421f9f8fe9759692302905268f4036b287adcb/src/gmt_init.c#L393-L394

Details to address:

1. Should we have separate {X} and {Y} entries, or a single {XY} entry?
2. Do we want a) full documentation from https://docs.generic-mapping-tools.org/6.1/gmt.html#xy-full, or b) just the simplified shortened docsting found at https://docs.generic-mapping-tools.org/6.1/std-opts.html?

See related issue at #380. This PR will be a part of #620 too.

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.

[seisman]

1. Should we have separate {X} and {Y} entries, or a single {XY} entry?

A single {XY} entry looks better.

2. Do we want a) full documentation from https://docs.generic-mapping-tools.org/6.1/gmt.html#xy-full, or b) just the simplified shortened docsting found at https://docs.generic-mapping-tools.org/6.1/std-opts.html?

All common aliases (e.g., region, projection) are using a simplified docstring. We should follow the convention for X and Y, but we may need to find a better way linking to the full documentation.

[weiji14]

1. Do we want a) full documentation from https://docs.generic-mapping-tools.org/6.1/gmt.html#xy-full, or b) just the simplified shortened docsting found at https://docs.generic-mapping-tools.org/6.1/std-opts.html?

All common aliases (e.g., region, projection) are using a simplified docstring. We should follow the convention for X and Y, but we may need to find a better way linking to the full documentation.

Ok, sticking with the simplified docstring, and added a link to the full documentation. Below is how it looks like rendered, let me know if we should expand this a little bit more: