Change some docs organization and titles

[mtdowling]

This change makes some minor changes to how the guides section of the docs is laid out.

1. Rather than group the Gradle plugin and smithy-build.json under building models, they are now top-level docs alongside the top-level CLI docs. This removes the incongruence of grouping Gradle with the config docs while not including the CLI. I've also renamed the docs on the config file to smithy-build.json. Redirects were added to maintain existing links.
2. Docs for converting to OpenAPI, converting from 1.0 to 2.0, and converting to CloudFormation resource schemas were moved under a sub-guide called Model Translations. This moves these very specific and probably ever growing category of docs into their our guide. Redirects were added as well.
3. Added links to the CLI and smithy-examples to the quickstart.
4. Renamed Model Linters to Linting Models.
5. Renaming "Using Smithy Code Generation" to the less verbose title of "Code Generation". This shouldn't confuse people with how to build a code generator since that's called "Creating a Code Generator".

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