docs: kv, db, and cache updates

[mattmaribojoc]

• Adds sections to the KV, DB, and Cache docs that add context for common use cases/questions. This info is spread throughout Cloudflare/Nitro/Unstorage docs - so to smooth the learning pathway, porting some of that information here would help
• Adds Pricing sections to each page of the features docs (where applicable)
• Reorders KV page headings to follow the ## hubFeature() and ### binding.fn() format

Documentation for:

• Differences between KV and DB consistency model
• Page level caching with nitro.routeRules
• Nitro Caching defaults to swr and must be passed swr: false for expired values to be removed from cache storage
• Creating a table through DB migrations
• JSON queries through the D1 client

Example of per-page pricing information

[pkg-pr-new]

Open in Stackblitz

npm i https://pkg.pr.new/nuxt-hub/core/@nuxthub/core@454

commit: 592aaa1

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