General documentation markdown cleanup

[siddhesh]

The ported markdown documentation attempts to glean technical documentation that is relevant to moonjit such as installation and execution instructions, API, etc. It does not have benchmark numbers, sponsorship acknowledgements or contact details since they are not relevant to moonjit.

In the process however, a number of cross-links have gone missing and the port also may have errors, formatting and factual.

One needs to review the documentation, comparing with reality (i.e. actually verifying if it works) and also comparing with documentation in doc.luajit to fix any correctness issues and add cross links wherever relevant.