ci: github actions

[LeoDog896]

This adds GitHub actions support and deployment (fixes #20), with the following:

• pest-parser/book submodule
• GitHub deployment action
• Integrates mdbook into taskfile

Currently testing out actions on my own branch before I undraft this PR.
(In the process, had some branch mixups as I'm trying to be on the master branch to test out CI.)

[LeoDog896]

Seems to work well: https://leodog896.github.io/site/

[LeoDog896]

I'm having some trouble trying to publish mdbook under /book. It seems the github pages example wasn't great since I was already under a subdirectory, but going to /book locally shows broken links. How did y'all do it when publishing?

[tomtau]

I just ran mdbook build inside the book repo's directory and copied the book output directory to https://github.com/pest-parser/pest-parser.github.io/tree/master/book

[LeoDog896]

The issue was on me - I was using an NPM library, serve, which was messing with path resolution. The python dev server works wonderfully.

For ease of development, I've separated the tasks to cache wasm-pack as wasm-pack doesn't do this yet.

Otherwise, the PR is fully ready to go!

[tomtau]

the GH actions seem ok, I'm just trying to figure out what I should configure before merging it:

1. the environment
2. the custom domain? I don't see this setting on this repo (unlike for pest-parser.github.io) and not sure if there are some extra DNS records needed to be added (I don't control the domain)

[LeoDog896]

1. CNAME needs to be added (example from pest-parser.github.io)

2. In Settings -> Pages, change Source to Github Actions
   

3. (Your domain is most likely verified, since you use it already. If it's not, follow these instructions): Since pest-parser is an organization, domains are verified on the organization-level. So, in your Organization Settings, go to Pages , and then the domain owner needs to add the TXT records.

4. Back in Settings -> Pages, add your domain.