Split CI into multiple yaml files

[coretl]

Context

The existing monolithic CI had some problems:

• We want some parts to be optional, which required templated CI which then couldn't be easily tested
• We wanted to reuse some parts of the CI in the template repo
• It was long and hard to read
• It was split into multiple top level jobs

Decision

Break it into multiple reusable workflows that are called from:

• A single ci workflow that runs on push, and tests, builds, and adds artifacts to releases
• A single periodic workflow that runs once a week and checks links aren't broken

Fixes #39