Add autodoc website

This adds a `zig build docs` step that builds the documentation website
and writes it to zig-out/docs.

It further includes a GitHub Workflow that publishes this website
to GitHub Pages. The GitHub Workflow is divided into two jobs:

- build: builds the documentation and uploads it
- publish: downloads the documentation and publishes it

These are separate jobs to minimize permissions available
to the build job.

This workflow runs on two events:

- after every push to master
- `workflow_dispatch`: this allows manually running the workflow
  from its *Actions* page if something went wrong

---

**Important pre-merge steps:**

If this PR is accepted, the following steps should be taken
before merging the PR:

1. Go to **Settings** for the repository
2. Select **Pages** on the left under *Code and automation*
3. Under *Build and deployment* set **Source** to **GitHub Actions**
4. Merge the PR.

If the steps are missed, the PR will merge just fine,
but the docs job will fail immediately on merge.
This can be remedied by following steps 1-3 above,
and either adding a new commit on master,
or manually firing the workflow from the Actions > API Reference page.

1 files changed, 8 insertions, 0 deletions

diff --git a/build.zig b/build.zig
index bcf600e..ead9282 100644
--- a/build.zig
+++ b/build.zig
@@ -35,6 +35,14 @@ pub fn build(b: *std.Build) void {
         example_step.dependOn(&install_example.step);
     }
 
+    const docs_step = b.step("docs", "Generate docs.");
+    const install_docs = b.addInstallDirectory(.{
+        .source_dir = tests.getEmittedDocs(),
+        .install_dir = .prefix,
+        .install_subdir = "docs",
+    });
+    docs_step.dependOn(&install_docs.step);
+
     const readme_step = b.step("readme", "Remake README.");
     const readme = readMeStep(b);
     readme.dependOn(example_step);