Combine the Oblique Mercator projection pages into one page

[yvonnefroehlich]

Description of proposed changes

This PR aims to combine the three documentation pages for the Oblique Mercator projection (Oa, Ob, Oc) into one common page. For the idea / context see https://github.com/GenericMappingTools/pygmt/pull/3407#issuecomment-2367022084

Preveiw:

• https://pygmt-dev--3451.org.readthedocs.build/en/3451/projections/cyl/cyl_oblique_mercator.html#sphx-glr-projections-cyl-cyl-oblique-mercator-py
• https://pygmt-dev--3451.org.readthedocs.build/en/3451/projections/index.html#cylindric-projections
• https://pygmt-dev--3451.org.readthedocs.build/en/3451/techref/projections.html#gmt-map-projections

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.

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• /format: automatically format and lint the code

[yvonnefroehlich]

Do we also want to combine the Cartesian projection / transformation (linear, logarithmic, and exponential) into one page?

[seisman]

Do we also want to combine the Cartesian projection / transformation (linear, logarithmic, and exponential) into one page?

I prefer separate pages for them so that we can show all of them in the gallery page.