docs/manual: convert release notes to markdown

docs/release-notes/rl-0.6.md

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+# Release 0.6 {#sec-release-0.6}
+
+Release notes for release 0.6
+
+## Changelog {#sec-release-0.6-changelog}
+
+[ksonj](https://github.com/ksonj):
+
+- Add Terraform language support
+
+[horriblename](https://github.com/horriblename):
+
+- Fixed empty winbar when breadcrumbs are disabled
+
+[notashelf](https://github.com/notashelf):
+
+- Finished moving to `nixosOptionsDoc` in the documentation and changelog. We are fully free of asciidoc now
+
+- Bumped plugin inputs to their latest versions