Consistent URL links when reference images in the GMT documentation

[seisman]

Sometimes we need to reference images or documentation in the GMT documentation. Usually, we use the gmt-docs directive, but sometimes we can't and must use full URLs.

The URLs are inconsistently pointing to 6.5, 5.6, latest, and dev:

pygmt/encodings.py:19:- GMT documentation: https://docs.generic-mapping-tools.org/dev/reference/octal-codes.html
pygmt/tests/test_text.py:297:    https://docs.generic-mapping-tools.org/latest/gallery/ex23.html
pygmt/src/nearneighbor.py:65:    .. figure:: https://docs.generic-mapping-tools.org/dev/_images/GMT_nearneighbor.png
pygmt/params/position.py:21:    .. figure:: https://docs.generic-mapping-tools.org/dev/_images/GMT_anchor.png
pygmt/params/pattern.py:29:    .. figure:: https://docs.generic-mapping-tools.org/6.5/_images/GMT_App_E.png
examples/projections/cyl/cyl_universal_transverse_mercator.py:17:.. figure:: https://docs.generic-mapping-tools.org/latest/_images/GMT_utm_zones.png

I think we should use a consistent URL. I prefer to use a specific version (e.g., 6.6), so that any changes in the GMT documentation won't break our documentation suddenly.

[yvonnefroehlich]

Consistency sounds good. I think GMT 6.6.0 is better than dev, since for older PyGMT releases, the GMT dev version could break the docs or make no sense due to later updates. But we should add updating these image URLs to the current GMT version to the release checklist to not forget this manually.

[seisman]

But we should add updating these image URLs to the current GMT version to the release checklist to not forget this manually.

Can be added to https://github.com/GenericMappingTools/pygmt/blob/main/.github/ISSUE_TEMPLATE/5-bump_gmt_checklist.md.