Add option for custom child-template repo

In some cases, people want to use the nf-core template and associated best practices (and updates), but constantly need to apply the same set of modifications when creating new pipelines. In this situation, it would be nice to be able to supply a "child template" that overwrites specific desired template files, and/or adds additional ones.

Template overrides are a pretty standard mechanism, see examples such as mkdocs, MultiQC and more.

Workflow should be:

1. Create a new repository for the custom template. Add any files that you want to overwrite.
   • For example, if you want a custom readme but everything else the same, create just a README.md file and nothing else
2. Run nf-core create --template https://github.com/myorg/mytemplate
3. The create command generates a pipeline as normal. It then renders the custom repo template afterwards, and copies these generated files on top of the vanilla pipeline output.
4. The template git repo is noted in .nf-core.yml so that the same process can be repeated for pipeline syncs (and even linting?)

Will need additional docs to describe the various template variables that are available for use, such as {{ name }} etc.

Start by tracing the nf-core create command's template-rendering path and how .nf-core.yml is read during pipeline syncs. Check whether linting also consumes that configuration. Done means a child template repository can overlay or add generated files, is recorded in .nf-core.yml, and the behavior is documented with available template variables.