Refactor doc to restructure and improve #1943

[DennisClark]

This PR is for:

• VCIO-next: Improve and refactor documentation for clarity #1943

@TG1999 and @keshav-space I have done the basic restructuring to format the rtd index to be consistent with
https://github.com/aboutcode-org/scancode.io/blob/main/docs/index.rst

Can you please check it over for syntax, etc.

The revised index.rst also points to these files, which I copied to docs/source (but have not removed them from the main level yet):
PIPELINES-AVID.rst
SOURCES.rst
api_v3_usage.rst

We'll probably want to add some explanatory text for the first two to give them some context; this is still a work-in-progress.

[TG1999]

We don't have this rst file yet.

[TG1999]

@DennisClark we have some errors while building docs, we need a toctree. Also there are few files where we need to add a title. Thanks!

[TG1999]

@DennisClark you can add a toctree like this

.. toctree::
   :maxdepth: 2
   :caption: Getting Started

   introduction
   installation
   user-interface
   api
   api-admin
   api_v3_usage

.. toctree::
   :maxdepth: 2
   :caption: Tutorials

   tutorial_add_importer_pipeline
   tutorial_add_improver_pipeline

.. toctree::
   :maxdepth: 2
   :caption: Reference

   reference_importer_overview
   reference_improver_overview
   reference_framework_overview
   reference_model_overview
   command-line-interface
   importers_link
   PIPELINES-AVID
   SOURCES

.. toctree::
   :maxdepth: 1
   :caption: Misc

   faq
   misc
   soc_gsoc21

And add title in file which are missing title like this

PIPELINES AVID
==============

Also verify errors with make doc command. Let me know if you want to me take a look on any particular thing. Thanks!

[DennisClark]

@TG1999 looks like you've got the solution all figured out! Please go ahead with the change if you have time, thanks.