Expand the documentation for `specs/protocol/tx_format.md` to include descriptions of each section

[Braqzen]

Summary

Reading the tx_format.md documentation is confusing because none of the sections explain what any of the sections are, why they are important, where they come from, how they are generated, where they are used etc.

Taking a look at tx_validity.md it is much clearer to follow what is going on because there is some context about each section. This allows for a bigger picture understanding of all of the pieces rather than a section which purely describes how something may be invalid and a 1 short line description of each key.