Document filters

[mwd102]

Docs for search filters are pretty sparse. I notice not all dbs have support which is fine - it might be nice to get a table comparing db features beyond just hybrid. Anyway, I see I can use jsonb to do some powerful filtering in postgres but I had to dig about to learn that. Also I'd like these filters to be exposed to the playground UI so I can apply them e.g set a max age of a chunk. If the agents could be given a filter schema and intelligently apply them even better!