Restructure documentation and add or streamline beginner-friendly examples

[rhugonnet]

Final PR description

This PR includes:

• A restructuration of documentation for clearer sections "Getting started", "Background", "Features", "Gallery of examples",
• An update of the landing page,
• The addition of beginner pages on "About xDEM", "How to install" and "Quick start",
• A streamlining of existing examples to be more beginner-friendly,
• A change in dev-environment.yml to pass tests dependant on GeoUtils without requiring a new release. See new issue Switch back from git-based to release-based GeoUtils in dev-environment #268 to switch back once our packages are mature enough.

New documentation can be viewed here: https://xdem-rhugonnet.readthedocs.io/en/doc_struc/

Resolves #258

We merge this PR early to match @adehecq's EGU talk.
Similar objectives described below will be addressed in later PRs.

Points of issue in original PR

1. Structure of documentation (see Subsections of sphinx-gallery + Content headers for documentation #264)
2. Location of test data VS Location of gallery examples (see Subsections of sphinx-gallery + Content headers for documentation #264 and Split doc into "Getting started" and "Advanced guide" #258)
3. Structure of information in README (see Split and streamline information in README  #206)

[erikmannerfelt]

Wow, this is a fantastic improvement! I only have some very small comments.

Sorry for the delay

[rhugonnet]

I think this is perfect to have a more intuitive landing page + getting started examples for EGU 👍

We could merge this PR just for EGU, and advance further on the documentation (e.g. splitting of the examples) in a later one. Is that what you have in mind @adehecq ?

[rhugonnet]

When we talk about the package name we write "xDEM", and for the module sometimes xdem or xdem. Which one should we pick and stick to?
In the NumPy docs, looks like they stick to the package name "NumPy" without any kind of highlighting, and that the lower case, package-style naming numpy is only used with a submodule e.g. numpy.array

[adehecq]

I agree. I have no strict opinion on what we should use. In case of NumPy, the name is somewhat highlighted with the camel syntax. It feels a bit weird to write xdem in the text as if it was a regular word, so I like having a way to highlight it.
We agreed that lowercase is better than uppercase so that's why I went with the italic (xdem) and I think the code highlight (xdem) should be reserved for actual code so not in the text.
What do you think?

[rhugonnet]

Just looked at what is done in the docs of Matplotlib, NumPy, SciPy, PyTorch, GPyTorch, I'd do the same: use the "normal text" package syntax that is camel-case/acronym-coded.
For us, it would correspond to using "xDEM" in normal text everywhere (in a way, its capitalization would serve to highlight the name).

[rhugonnet]

Still time to change this later: to avoid the discrepancy between the xdem and xdem in the docs of this PR, I changed into xDEM everywhere to mirror the syntax used in big packages as described above. It will be easier to replace if we want to, because with the capitalized letters it doesn't mix with the code ocurrences :)

[adehecq]

Thanks a lot for making the final push !!