This PR does several things for the SDK docs:

1. Removes a redundant pages directory to reduce paths to mdx files
2. Introduces a reference section where we can document the schema of docs.yml
3. Removes the enterprise section and adds the self-hosted guide to the Guides section
4. Updates all the section names to include the relevant package managers (used to all be NPM)
5. Renames the top section to be Overview instead of Fern SDKs. Renames the first guide to be Introduction.
6. Updates all customer showcase to be links out to the showcase page (the links may change in the future to be more specific to each showcase).