Tweak GHA config to reduce bors time

So, "bors time" with the current config is slow a bit (~ 1h30m). We could reduce this by separating style check and docs generation, tweaking `max-parallel`, let's try it out!

Upload documentation to gh-pages from GHA

I missed this on #1918 

Only style check on PRs to reduce CI time

I think generating docs on PR jobs is overkill, just style check is enough.

Serial Experiments GitHub Actions

Let's start the party to close #1582.

BuildChannelsMacOS, SemverLinux, and SemverMacOS

StyleAndDocs, BuildChannelsLinux, and DockerSwitch

Fix {aarch64,arm}-android targets' CI

Android: Add VMADDR_ constants.

pthread_getattr_np is not present on illumos

Fix bootstrap on redox

Avoid attempting to enable the static_nobundle feature twice, which
results in rustc error E0636.