Add "/runbooks" page to github_branch

The runbooks source files are in the runbooks *sub-directory* of the
cloud-platform repo, not the repo root (which seems to be the assumption
that the tech-docs gem is making).

This PR changes the "github_branch" from "main" to "main/runbooks". This
is a bit of a hack, but it seems to work, and fixes the "View Source"
links in a local development instance of the site.

runbooks/config/tech-docs.yml

@@ -36,7 +36,7 @@ prevent_indexing: true
 
 show_contribution_banner: true
 github_repo: ministryofjustice/cloud-platform
-github_branch: "main"
+github_branch: "main/runbooks"
 
 owner_slack_workspace: mojdt
 default_owner_slack: '#cloud-platform'