docs: improve databases documentation

[itaysk]

Description

• document the recently added db fallback options
• added checks db to db documentation
• removed unnecessary and and confusing section from air-gapped document

[itaysk]

the release note says: "Databases are downloaded in priority order until one is successful". does this mean that trivy will keep cycling through list "until one is successful"? or if it reaches the end it will error out

[itaysk]

I wanted to incorporate the rate limiting mitigation steps post into the documentation, but after re-reading the docs I see that there's a growing overlap between the "advanced netwrok scenatios" (aka air-gap) doc, and the "Database cofiguration" doc. The way I thought about it is that the configuration document only describes the flags avaialble and technical usage, and the air gap doc talks more about the scenario and provides recommendations. Does this make sense to you? if so, I'll need to refactor both documents accordingly

[itaysk]

I ended up reorganizing the configuration/db doc and the air-gapped doc, such that the db doc now documents everything related to trivy's databases, including purpose, requirements, and configuration options, and the air-gapped explains how to setup trivy in a network constraint environment. I also incorporated parts from the yet-to-be-published db throttling announcement into the relevant documents.

[simar7]

@itaysk it lgtm for this iteration. I think we can merge this for now and update in subsequent PRs if more changes are still needed. WDYT @knqyf263?

[itaysk]

thanks for your review @wkoot !