Autodocs

[graebm]

Automated update the API documentation published to https://aws.github.io/aws-iot-device-sdk-cpp-v2/

Issue:

• Docs weren't updated unless someone remembered to do it manually.
• Manually updating docs whenever you update code pollutes the diff, making it hard to review
• Having HTML docs in main branch can cause issues.
  • Bloats size.
  • Generated docs might contain licenses we don't want to be redistributing.

Description of changes:

• Docs are kept in docs branch now
  • New docs.yml Github Workflow updates the docs branch whenever the main branch changes.
  • New docs-check job in ci.yml checks for warnings/errors in pull-requests.
  • delete the docs/ folder in main and use .gitignore to prevent someone from accidentally resurrecting it.

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