ci: Check all crate documentation in addition to target-specific ndk

[MarijnS95]

Following #106 (review)

Only ndk docs are built for all android targets but the rest of the documentation remains untested. Especially now that we're expanding to provide more complete docs for ndk-build and cargo-apk should everything be tested, to make sure that things like intra-doc links remain in working order.

TODO

The ndk docs build successfully here thanks to the test feature (enabled by --all-features); does it still make a lot of sense to build-test the documentation for every target individually?

[msiglreith]

does it still make a lot of sense to build-test the documentation for every target individually?

I don't expect any target specific doc parts tbh. Doing it once for one would be fine as well imo.

[MarijnS95]

@msiglreith I guess it's safe to remove then, and have the documentation depend on the test flag (and iirc a cfg(doc) feature is on its way, though it was disabled again on a repo I was contributing to due to it not being propagated to all crates properly).

[MarijnS95]

@msiglreith Removed it :)

Btw we have a lot of build steps now that do not use --target but are repeatedly ran for every rust-target in the matrix, wasting a ton of time and resources. As soon as this is merged I want to separate those out into two jobs that can run in parallel: some that build/test the host tools and docs, while the other simply only installs cargo-apk (bit of overlap with the build in the first job) followed by just the target-specific ndk check and apk build.