DOC: Combine the modern and classic ReST files into a single file

For almost all plotting modules, we usually have two separate ReST files for modern and classic modes, respectively. For example, we have `image.rst` and `psimage.rst` and `gmtlogo.rst` and `gmtlogo-classic.rst`.

More ReST files usually mean more maintenance burden, especially sometimes we may fix or update the modern mode examples in one file, but forget to update the corresponding examples in another file.

So, I guess it makes more sense to have only one documentation file for a single module. Usually, the differences of modern and classic ReST files are:

- `-K`/`-O`/`-P` is only available in classic mode
- Examples have different syntax.

For point 1, this can be addressed by emphasizing the these options are classic-mode only. For point 2, modern/classic modes can be switched using sphinx-design's tab directive (https://sphinx-design.readthedocs.io/en/furo-theme/tabs.html).

#8487 is an example PR showing how it works.

The title:


Classic-only options:


Modern-mode example:


Classic-mode examples (after clicking the "Classic mode" Tab: