Print a warning when main/master confusion detected

[mortenpi]

Print something like this in deploydocs

┌ Info: 
│ Buildkite config:
│   Commit branch: "main"
│   Pull request: "false"
│   Commit tag: ""
│ Detected build type: devbranch
│ - ✘ ENV["BUILDKITE_BRANCH"] matches devbranch="master"
│ - ✔ ENV["DOCUMENTER_KEY"] exists
└ Deploying to folder "dev": ✘
┌ Warning: Possible deploydocs() misconfiguration: main vs master
│ Documenter's configured primary development branch (`devbranch`) is "master", but the
│ current branch ($BUILDKITE_BRANCH) is "main". This can happen because Documenter uses
│ GitHub's old default primary branch name as the default value for `devbranch`.
│ 
│ If your primary development branch is 'main', you must explicitly pass `devbranch = "main"`
│ to deploydocs.
│ 
│ See #1443 for more discussion: https://github.com/JuliaDocs/Documenter.jl/issues/1443
└ @ Documenter ~/Julia/JuliaDocs/Documenter/src/deployconfig.jl:792

on Travis, GitHub Actions and BuildKite if the branch is main, but devbranch is not set. It doesn't detect the case where the user explicitly set devbranch = "master", but I think we can live with that (not worth the complexity).

Related to #1443.