Converted docstrings from reST to Google style

[forman]

• Converted docstrings from reST to Google style
• Changed devguide accordingly using docconvert
• Adjusted Sphinx configuration, which is now using sphinx.ext.napoleon for API doc generation

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[thomasstorm]

Tiny suggestion, otherwise looking fine!

[pont-us]

Approved, but note that docconvert has missed many functions (at least 41 by my count) -- list of files with remaining rST docstrings appended below.

However, I don't see this as a barrier to merging. There's no quick workaround to convert the remaining rST string (at least, I couldn't persuade docconvert to do anything with them), and converting them all by hand would be very time-consuming. So I think we should merge this PR now and fix the remaining strings as and when we come across them during other work. Sphinx + napoleon handles mixtures of both docstring styles, so the rendered documentation is fine in any case.

xcube/core/compute.py
xcube/core/level.py
xcube/core/_tile2.py
xcube/core/subsampling.py
xcube/core/resampling/spatial.py
xcube/core/reproject.py
xcube/core/gen2/generator.py
xcube/core/zarrstore/generic.py
xcube/core/update.py
xcube/core/gridmapping/base.py
xcube/core/timeslice.py
xcube/core/store/fs/accessor.py
xcube/core/store/fs/store.py
xcube/core/store/datatype.py
xcube/core/store/store.py
xcube/core/geom.py
xcube/webapi/compute/context.py
xcube/webapi/ows/wmts/controllers.py
xcube/util/expression.py
xcube/util/dask.py
xcube/util/cache.py
xcube/util/temp.py
xcube/util/extend.py
xcube/util/jsonschema.py
xcube/util/jsonencoder.py
xcube/server/asyncexec.py
xcube/server/webservers/tornado.py