Support explicit ordering of sphinx galleries

[oruebel]

Motivation

This PR was inspired by the recent PR #1495. This PR make the following changes:

• Generalize the CustomSphinxGallerySectionSortKey to allow explicit ordering of galleries for any sections while still providing the default alphabetical sort for any galleries that are not explicitly listed.
• Define explicit order for the domain-specific tutorials to group and sort tutorials based on domain, such that, e.g., all icephys tutorials appear together and tutorials appear in order of domain (ecephys, ophys, icephys).
• Add How-to guide with brief instructions on how to create a new tutorial

How to test the behavior?

Build the docs

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1504 (6a9741d) into dev (3338522) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1504   +/-   ##
=======================================
  Coverage   78.64%   78.64%           
=======================================
  Files          37       37           
  Lines        2777     2777           
  Branches      517      517           
=======================================
  Hits         2184     2184           
  Misses        510      510           
  Partials       83       83           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3338522...6a9741d. Read the comment docs.