Running doc only PRs through only the doctest CI

[ViralBShah]

I wonder if we can adopt a PR submission workflow, where PR submitters can pick the nature of their PR. For example, if it is doc only, they say so during PR submission, which can assign a label and the CI system can then just use that label to run only doctests related CI.

cc @DilumAluthge @staticfloat

[DilumAluthge]

Instead of having the user pick a label, I think it will be better if:

1. Our CI system computes the list of files modified in the PR.
2. If only doc-related files are modified, the CI system automatically skips the non-doctest tests.

[ViralBShah]

It is not easy to do that, because most of the doc changes are actually in .jl files. Checking if only the manual changed would miss out the API docs changes that go into the .jl files.

[DilumAluthge]

Cross-ref JuliaCI/julia-buildkite#243