docs/examples

[d-v-b]

This PR changes the layout of our top-level examples directory to be a directory-of-directories, where each sub-directory contains code + a README.md, which will make browsing these examples on github friendlier.

I also add a page to the user guide called examples that references the source code + readmes for the different examples. Right now we only have 1 example, but I would like to eventually add more, e.g. to demonstrate running benchmarks or integrating zarr with other tools, like visualization tooling.

[d-v-b]

closes #3501

[maxrjones]

I think these would be easier to find if the examples are separate from user-guide, rather than nested, but support merging if you have a strong preference for the current layout

[d-v-b]

I think these would be easier to find if the examples are separate from user-guide, rather than nested, but support merging if you have a strong preference for the current layout

no strong preference! i took your suggestion.