Docs: Add details related to testing in docs

[kmr-srbh]

The problem

The current information related to testing is confusing and unclear. It does not guide one on what the various types of tests are and how to write and register them.

The fix

This PR does the following things:

• Add details about the various types of tests, where they are located and why they are required.
• Add details about the procedure to write integration tests and update references.
• Add details about the procedure to write error tests and update references.

Minor: Fix indentation in the README and installation docs for visual clarity.

Planned:

• Add details on compile time tests and their requirement. (guidance required)
• Add details on troubleshooting failing CI tests.

[kmr-srbh]

@Shaikh-Ubaid @Thirumalai-Shaktivel I request you to check this data and verify the correctness. I request you to provide me more information regarding compile time tests so that the section can be complete.

[ubaidsk]

It looks like there are many indentation changes. It is hard for me to see/understand what changes were made.

[kmr-srbh]

@Shaikh-Ubaid I request you to try the rich diff. In all, the indentation related to installation instructions were reduced; they are the same for the installation docs and the README. The section on testing was improved.