Publish rune docs to netlify

[Qkessler]

Change Summary

As discussed on #48, we see valuable to publish the documentation to somewhere consumable, for future contributors to see the documentation without actually having the barrier of running cargo doc --workspace on their machines.

I tried different things and ended up choosing netlify. Netlify has a very powerful build system and it's running on Ubuntu, but it did not allow for apt-get installs. Since we need pango and gdk to run cargo doc on the crate, moved to Github actions.

As part of the same PR, I fixed all the warnings on cargo doc, making sure that the documentation is correctly published. The trick to deploy cargo doc is to deploy target/doc and then pair that with a redirect of the main page (configured rune-rs.netlify.app on my own account) to /rune which (as the main crate) has the info about the subcrates as well.

IMPORTANT: For this to work, you need to do the following steps. I would do the steps myself, but I imagined you would like to have control of the app itself, as the author of the project:

1. Create a Netlify account if you don't have any.
2. Create a new site with the Rune project on Github.
3. Note down the site id.
4. Change the site name to something like rune-rs, for https://rune-rs.netlify.app to work. I'll remove my site with the same name to avoid conflicts.
5. Go to your profile, User Settings, Applications and Generate a No Expiry (or to be expired, although that will need changing the secrets whenever it expires again) Personal Access Token and note it down.
6. Go to the SECRETS configuration of this repo.
7. With the site id you noted down, add the NETLIFY_SITE_ID secret.
8. With the personal access token you noted down, add the NETLIFY_AUTH_TOKEN secret.

The site will be published on any push to master, no need to publish on PRs!

Risks associated with this change

I don't really see a risk per se, other than needing to configure Netlify with the secrets of the Github repo.

Testing

Published the app on all my commits on the docs branch on my fork. Additionally, if you see the commit that is linked on this PR, the CI on that commit has the deployed app (running the CI I'm proposing as part of this PR).

[Qkessler]

Since I removed my own rune-rs site, you can already use that name on the steps above. If you are interested in checking how the documentation is published, I still published my site to https://qkessler-rune-rs.netlify.app. Enjoy!

[CeleritasCelery]

Thanks for working on this! I tried to follow your steps, but we will see how it actually goes.

[CeleritasCelery]

Looks like it worked! https://rune-rs.netlify.app

I am wondering if we should change most of the pub(crate) items to just pub? It won't make a difference semantically because this is not a library, but it will make the docs look cleaner.

[Qkessler]

Yeah, I did that on my core refactor as well.