Enable docs build on ReadTheDocs

[khaeru]

In this PR, I've done the following:

• add the necessary pieces for the documentation to be built on ReadTheDocs,
• change the theme to the RTD default theme with the IIASA logo,
• rearrange and simplify duplicated contents, especially in the user/contributor guidelines and FAQ.

I have temporarily (i.e. this will not be the final URL) enabled the build for this branch (feature/rtd), which is available at:

https://message-ix.readthedocs.io/en/feature-rtd/

• Merge Improve documentation #138 and ensure build still works.
• Set up CNAME or redirects from IIASA ENE domains & corresponding RTD config.
• Consider subprojects or other configuration for building docs of MESSAGE-GLOBIOM and ixmp separately → don't use
• PR checklist:
  • Tests added (n/a)
  • Documentation added (n/a)
  • Description in RELEASE_NOTES.md added

Closes

• Enable tutorials to be built on RTD #155
• rendering of MESSAGEix documentation on Firefox and EDGE #157

Separately

If we want to enable RTD hosting of older versions (e.g. v1.0.0, v1.1.0), then we will need to:

1. Establish maintenance branches for each version.
2. Add a minimal commit to each branch with changes necessary for the RTD build to work.

[khaeru]

• Set up CNAME from IIASA ENE domains & corresponding RTD config.

This was discussed in today's development meeting, work is on hold pending a decision…

[khaeru]

Last thing pending is a decision from @volker-krey on what domain/path these will live under.

[gidden]

Looks great @khaeru. What did we decide on migrating tutorials to static doc pages? I know gams was the pain point, but did we come to a conclusion?

[khaeru]

@gidden I think the idea was to:

1. Install GAMS as an optional target of message_ix/ixmp.
2. Do this on RTD.
3. Render the tutorials within Sphinx, either from ipynbs or inline content.

I would personally prefer static docs pages, since that seems more in line with what's done by other Pydata packages. Either way, that's a separate can of worms, so we can open a new issue…

[gidden]

Agreed. Ok, so we have the scripts to do so on CI, but I would need to read up on how to make setup.py do something like this specifically for a target. Perhaps a custom Install class?

[khaeru]

Either way, that's a separate can of worms, so we can open a new issue…

Opened #165 for that discussion.

[khaeru]

Okay @gidden I think this is ready to merge, if you do.

I will make another PR if needed to implement the final domain/URL scheme.

[gidden]

Amazing work, thanks @khaeru!