Port over JSON builder avoidance from pydoc

[agjohnson]

Pydoc gets around this by building the JSON output into Sphinx. Port this over.

[ericholscher]

The existing logic is:

• RTD: https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/projects/tasks.py#L616-L620
• RTD: https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/doc_builder/backends/sphinx.py#L242
• RTD: https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/projects/tasks.py#L730-L735
• Pydoc https://github.com/rtfd/pydoc.io/blob/master/pydoc/sphinx/__init__.py#L32-L55

The quick version is:

• Currently we run an entire second build to output JSON from Sphinx, in order to support search
• We should output the JSON as an artifact of the build process
• This might hit some edge cases in building people's docs, but it would save around 20-25% of our build resources, which would be a huge win for us

There will likely be some issues with weird encoding and other things, but I'd really like this to get some focus here soon.

[ericholscher]

@safwanrahman you might find this and the attached PR's interesting!