pliron-llvm docs.rs build is broken

[vaivaswatha]

For a while now, because pliron-llvm requires a local LLVM installation, and docs.rs does not have this, the doc build has been broken.

Reference: https://docs.rs/crate/pliron-llvm/0.9.0/builds

[vaivaswatha]

Docs for master are now published on GH Pages, updated on every commit.

[vaivaswatha]

This requires adding the right LLVM version as an apt-get dependence within the doc.rs scripts, which must be done by testing and creating a PR there. I'm not sure how practical that is given that we keep moving LLVM versions and they may have an older version of Ubuntu that might not have the right version.

Ref: https://forge.rust-lang.org/docs-rs/add-dependencies.html