On `tabularray`'s implementation manual

[peaR-red]

As per parent issue #641, both parts of the tabularray documentation should be separated and revamped. The problem with the current documentation for the source code is its lack of clarity, much like the user's guide. Comments are usually not that descriptive and does not fully capture the essence of the implementation. This makes it difficult for future contributors and maintainers to parse the underlying package internals.