Expand doc on deal-fitlering via external hooks

[ribasushi]

@johnnymatthews @jnthnvctr so... this is ready for review but is also somewhat of a draft:

• I scaled back the lean on "storage-deal filtering" and extended things in the general doc on filtering.
• The functionality added back in allow storage miners to manage a blocklist of piece CIDs lotus#2069 was mid-documented ( it only works with PieceCIDs ) and I'd argue we need to go further and remove it entirely: it is of little utility :(
• I would personally find examples of the JSON that is being fed to the programs very useful, but am not sure where it would fit nicely. Examples of both can be seen here: https://github.com/Murmuration-Labs/bitscreen/tree/45a84483c5a369c6fe6db90723aee11909b33ab4/testdata

[johnnymatthews]

Sorry for the delay in getting to this. Made some suggestions, mostly just formatting and grammar fixes.

Merge when ready.