Fix Doc typos in ThreatModel & Added README on how to build docs

[kskarthik]

Added build instructions for the docs & a README file in the /documentation section pointing it.

Before you submit a pull request, please make sure:

• The pull request should include a description of the problem you're trying to solve
• The pull request should include overview of the suggested solution
• If the pull requests changes current behavior, reasons why your solution is better.
• The proposed code should be fully functional
• The proposed code should contain tests relevant to prove is functionality
• The proposed tests should ensure significative code coverage
• All new and existing tests should pass

[evilaliv3]

Thank you @kskarthik !

I've manually integrated some of your fixes and assigned the commit to you to maintain your credit.

9a405c0

[evilaliv3]

Same made for:

• d64483b
• 575037b