docs: improve documentation prior to oxlint beta release

[camchenry]

We have not updated the documentation much to reflect the large number of new features released over the last year. Before doing a beta release, we should ensure that the documentation has guides on how to use all of the features available.

• better general docs on configuration file specification
• oxlint-disable-* and inline configuration
• nested configuration
• extends
• overrides
• how to use plugins
• different formatting / outputs (--format)

[Boshen]

For nested configuration, we need to state that the user needs to replace the previous -c .oxlintrc.json argument with --experimental-nested-config. I got confused by this for a second.

[Boshen]

The behavior of using extends is really subtle. People may end up not applying the rules they wanted. We need to emphasize this a bit in the doc, and also print some information after running `oxlint.

How does Biome / Ruff deal with extends?