fix: Use bootstrap version from template package if not defined locally

[gadenbuie]

Fixes #2440
Supercedes #2441
cc @maelle

Bootstrap versions can come from two places:

1. The local pkgdown config of the package being documented
2. The pkgdown config of a template package used by the package being documented

Unfortunately, they're interconnected: A local pkgdown config may set a Bootstrap version that then changes the template used by the template package. OR a template package may provide the Bootstrap version, not sepcificied in the local config. I missed this nuance in #2395.

This PR updates the Bootstrap version check to first call get_boostrap_version() on the local pkg config. Then we separately call get_boostrap_version() for the template. This function throws if both template.bootstrap and template.bslib.version are used in the same config. The separate calls let us adjust the instructions for fixing the problem depending on whether the invalid config is local or part of the template package. Local Bootstrap versions override template versions, providing an easy escape hatch if the template package has an invalid config.

I wanted to ensure that _pkgdown.yml config files follow the same structure regardless of whether they appear in a local package or in a template package. I've added tests around all of the behavior described above. I also added a new test helper local_pkgdown_template_pkg() to easily spin up a new templatepackage test package.

[hadley]

Thanks!