Deploy the dev version of docs to GitHub Pages

[yutannihilation]

Since docs.rs serves only the published version, providing dev docs on GitHub Pages is useful. This is not what every Rust crate does, but many packages does e.g. diesel and servo. You can view the example here: https://yutannihilation.github.io/extendr/extendr_api/

Some things I'm not sure are:

• Where to place the link to the docs: GitHub Pages doesn't have the badge
• How to generate the index page nicely: it seems rustdoc already has this functionality, but it's not available in stable cargo doc (c.f. Feature idea: cargo doc generating an index.html for workspaces, with a list of the workspace's crates rust-lang/cargo#8229 (comment)). Should I generate the docs by using nightly?

[clauswilke]

Do you need the packages llvm-3.9-dev and libclang-3.9-dev? On ubuntu-20.04, I found I don't need them, and don't need to set LIBCLANG_PATH either: https://github.com/clauswilke/helloextendr/blob/main/.github/workflows/R-CMD-check.yaml

(Note that ubuntu-latest is currently 18.04 but will soon switch to 20.04: actions/runner-images#1816.)

I think it's better to write action scripts that don't rely on specific versions of llvm/libclang, in particular if they're quite old.

[clauswilke]

Oh, and when ubuntu-latest switches over to 20.04, the script will break because llvm version 3.9 doesn't exist for 20.04.

[yutannihilation]

Thanks, I just copied these lines from from the existing YAML and didn't think about the details. Let's remove them.

[yutannihilation]

Confirmed the test passes without installing additional system libraries and LIBCLANG_PATH.

https://github.com/yutannihilation/extendr/runs/1471710150

[yutannihilation]

Ah, since the action on this repo is already setup in the era of 18.04, maybe I need to set ubuntu-20.04 explicitly at least once?

[clauswilke]

Thanks!

[clauswilke]

Now that I have merged this I see that the link to the development documentation in the README is a bit ugly. Maybe removed it from between the badges and just write it somewhere in the main body of the text, in a sentence: "The documentation for the latest development version is available here: ..."

[yutannihilation]

Ah, true. Sounds good to me. I'll prepare a pull request.