API doc tweaks [skip ci]

[gavinmacaulay]

Changes in this PR:

• use automodule (from the autodoc plugin) instead of the automodapi plugin to make the API docs
• add documentation for colormap

These changes are a bit more opinionated than my earlier doc PRs, so I give some context:

I've always found the echopype API documentation hard to follow, mainly because there were two ways to access the individual class/function documentation - the long unsorted list of function/member names under the API reference menu in the left sidebar of the docs and the same stuff under various headings in the right sidebar with a confusing Functions entry under each heading.

The changes in this PR remove the list of functions in the left sidebar and the Functions headings in the right sidebar. To me, at least, this makes for cleaner API docs that are also easier to look through for the class/function documentation that one is after.

I also noticed that the echopype-provided colormaps were only documented in the user guide part of the docs and not in the API part, so I added a colormap section with an enhanced docstring.

Using the Sphinx autodoc plugin instead of automodapi removes the etoc.toctree warning that is unavoidable when using automodapi with an external table of contents, so suppression of those warnings is also removed (resolves a comment in #1763). Automodapi is not actively developed compared to autodoc (see Development status in the readme), so not using it reduces potential future hassles.

These sort of changes to the API docs are what I was planning to attempt during the 2026 WGFAST echopype hackathon but never managed to do 😄.

[LOCEANlloydizard]

note because I'm thinking of this now and I have a feeling it's related to this issue re API doc, but I'll dig in a bit later! #1690