Add Ensign for developers page

[pdeziel]

Scope of changes

This adds a page to the documentation with an end-to-end pub/sub example.

The example code is available here: https://github.com/rotationalio/ensign-examples/tree/main/go/tweets

Fixes SC-12537

Type of change

• new feature
• bug fix
• documentation
• testing
• technical debt
• other (describe)

Acceptance criteria

Describe how reviewers can test this change to be sure that it works correctly. Add a checklist if possible.

Author checklist

• I have manually tested the change and/or added automation in the form of unit tests or integration tests
• I have updated the dependencies list
• I have recompiled and included new protocol buffers to reflect changes I made
• I have added new test fixtures as needed to support added tests
• Check this box if a reviewer can merge this pull request after approval (leave it unchecked if you want to do it yourself)
• I have moved the associated Shortcut story to "Ready for Review"

Reviewer(s) checklist

• Any new user-facing content that has been added for this PR has been QA'ed to ensure correct grammar, spelling, and understandability.
• Are there any TODOs in this PR that should be turned into stories?

[shortcut-integration]

This pull request has been linked to Shortcut Story #12537: Create E2E tutorial for Ensign MVP Developer user.

[rebeccabilbro]

Looks great @pdeziel, fantastic work! I added in a little publicity blurb to the examples/_index.md, hope that's ok!