Add smithy-docgen to the repository

[haydenbaker]

Background

• Adds smithy-docegen to the repository
• Previously, this was not easily possible with how the repository was structured and built
  • With Migrate build logic to conventions plugins #2484 and Require JDK17+ for development #2487, this is now (easily and cleanly) possible to do
• The core module was renamed from smithy-docgen-core to smithy-docgen to fit our conventional naming scheme
  • The actual module structure was not changed, everything is still under core (up for debate whether to change)
• Reviewers: Please double-check that surface area is as small as possible (everything that can be package-protected or marked as unstable is)

Testing

• Migrated the -tests package to a couple of simple integration (sanity-check) tests
  • The actual example usage (as a project) should be moved to smithy-examples

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.