Add Github action to build docs and upload them to S3.

[maciejwalkowiak]

:loudspeaker: Type of change

• [ ] Bugfix
• [x] New feature
• [ ] Enhancement
• [ ] Refactoring

:scroll: Description

Github action that builds & uploads docs for selected branches to S3.

Docs are uploaded to: https://docs.awspring.io/spring-cloud-aws/docs/2.2.5.BUILD-SNAPSHOT/reference/html/index.html

:bulb: Motivation and Context

Once project moves to awspring we need to publish docs ourselves.

This PR requires S3_AWS_SECRET_KEY and S3_AWS_ACCESS_KEY secrets set up in repository settings - that's why it's likely going to be merged only after we move to new organisation.

:green_heart: How did you test it?

:pencil: Checklist

• [x] I reviewed submitted code
• [ ] I added tests to verify changes
• [ ] I updated reference documentation to reflect the change
• [ ] All tests passing
• [ ] No breaking changes