Subsections of sphinx-gallery + Content headers for documentation

[rhugonnet]

We already have 12 gallery examples! Many more will come, and it's starting to be a bit messy!! 😟

Two points to improve this:

1. Organize the gallery in subsections. Thankfully, this is done super easily by the default behaviour of sphinx-gallery that splits galleries in sub-galleries if the examples/ folder has subfolders. Unfortunately, we already have the "Easton" and "Longyearbyen" subfolders in the example folder. Should we (A) move those somewhere else (tests/data or similar?)? Or (B) create a subfolder of examples/gallery/ that we make the parent of all sphinx-gallery?

2. Add content headers to the documentation. Right now we just concatenate sections without permanent headers. Essentially, we would have several toctree instead of a single one "Content" in our index.py, to split section, for example like the blue delineators of the Sphinx-Gallery documentation (https://sphinx-gallery.github.io/stable/index.html).
   For this, we should reflect on a tree, for instance:

Getting started

• What is xdem?
• Installing xdem
• First steps

Background

• Aligning digital elevation models
• Analysis of accuracy and precision

Features

• Vertical reference
• Terrain attributes
• Coregistration
• Bias correction
• Filtering
• Uncertainty analysis

Example gallery

• Beginner
• Advanced
• Expert

Reference

• API
• Development guidelines

Any input @erikmannerfelt @adehecq ? I'd be for option 1.A and 2. with something similar to this structure.
This is quite straightforward to do, but would be easier to implement once the terrain.py PR is merged.

[rhugonnet]

Forgot this was already opened partially in #258 by @adehecq! More issues to close 😁