Support 'base' in ConfigWrapper and Sphinx backend

[agjohnson]

As per the spec for https://github.com/rtfd/readthedocs-build, we should support the 'base' configuration element. This logic would need to be moved from the project model and into the sphinx backend, but this makes more sense anyways.

[Jigar3]

@agjohnson I want to work on this issue.
Can you give me more details regarding this

[RichardLitt]

@stsewd Would you be able to help with this?

[stsewd]

@RichardLitt @Jigar3 I'm not familiar with the https://github.com/rtfd/readthedocs-build project, but what I understand this would be a step for moving the build process to the other library. I'm going to take a look to the other project on the next days, so hope to be more helpful then.

[stsewd]

@RichardLitt well now I think I have some more experience with the other project (sorry for taking a long time).

@Jigar3 I recommend you reading the spec of the yaml file here https://github.com/rtfd/readthedocs-build/blob/master/docs/spec.rst, also https://docs.readthedocs.io/en/latest/yaml-config.html (#3685 contains more accurate docs) so you have a little more of context of what this issue is about.

From the spec:

base
The path to the root directory of the documentation that this config is about. That is usually the path that contains the conf.py file. It defaults to the directory that the readthedocs.yml file lives in. All commands for building the documentation will have the base set as working directory.

This is what the issue is about, respecting the base setting from the yaml file when executing build commands. Let me know if you need more information about the files that would need to be edited.

[stsewd]

@Jigar3 are you still interested or working on this issue?

[stsewd]

I think we can close this in favor of #4354

[stsewd]

Closing as this isn't on our current schema, and we have the new install key which accepts a path for each installation.