Bad Index in the documentation

[demidan]

Hi Kedar, Dmitry,
I've debugged a bit the empty Index problem that I've discussed with you before,
here is what I've found:

• it's not environmental problem -- it repeats well on my local machine as well as in our jobs
  depending on the commit:
  the (non empty) Index appeared in commit e742f3c,
  it become empty after commit 181835e
• I've compared the changes between them and understood the problem
  neither in conf.py (the e742f3c does not changed it)
  nor in index.rst (the 181835e does not change it).
• Actually it due to Kedar's change in overview.rst lines 39-95:
  Kedar have been removing extra indention across the codebase, and accidentaly moved 'Terms and Definitions' paragraphs out of scope of RST .. glossary:: directive.

So what's important that was the only place producing items for the Index -- not quite useful one :)
(and I havn't noticed it right away, silly me!)

@dzakhar, could you review the doc and think what it would make sense adding to the Index
(e.g. I expect that would be general notions as a tensor, operations on them, etc. as well as references to our C-level API data types, functions and defines).

@rkedar1, could you meanwhile look at Sphinx documentation (I guess domains is right page to start), and check what other directives (beside .. glossary::) are able to populate the Index?
After that we'll sync and decide what changes we should do to produce useful documentation Index.

Note: this is low priority issue.

Thanks,
Daniil