Rebuild manifest on merge of PR

[pvanheus]

The manifest (index.yml or index.json) should be rebuilt on merge of a PR. The plan is that this can be done using primaschema's build-manifest verb.

A Github Action that does this needs to be implemented:

1. Checkout repo
2. Build manifest
3. Added newly build manifest to repo (this is possible with actions/checkout)

[bede]

Agree. Currently the CI simply runs validate-recursive