feat: automate release on PR merge

[thiskevinwang]

What

This adds a GitHub action to run when PRs with branches like tfc-agent-changelog/1.2.6 are merged into main.

The version, will be derived from the branch name, and a GitHub release will be published

• v will be prepended, so v1.2.6

Why

This is in support of versioned-docs automation

Screenshots

Example from test repo

• https://github.com/thiskevinwang/workflows-test/actions/runs/2720725613

• Workflow summary:

  
• Created release:

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Name	Status	Preview	Updated
terraform-docs-agents	✅ Ready (Inspect)	Visit Preview	Jul 28, 2022 at 11:15PM (UTC)

[ryanuber]

Awesome work @thiskevinwang ! Thanks for tackling this!