fix(docs): restore docs.ethlambda.xyz custom domain on deploy (lambdaclass#399)

## Problem

https://docs.ethlambda.xyz/ returns **404**, even though the docs build
and deploy succeed and `https://lambdaclass.github.io/ethlambda/` serves
**200**.

### Root cause

| Check | Result |
|-------|--------|
| `lambdaclass.github.io/ethlambda/` | 200 (content live) |
| `docs.ethlambda.xyz` | 404 |
| DNS → GitHub Pages IPs | correct |
| Pages API `cname` field | `null` |
| `CNAME` file in `gh-pages` | absent |

The deploy step used `peaceiris/actions-gh-pages@v4` **without a
`cname:` parameter**, so every deploy republished `book/html` without a
`CNAME` file. The custom domain (originally set via the Pages UI, which
writes a `CNAME` file) got wiped by a later deploy, resetting Pages
`cname` to `null`. DNS still points the browser at GitHub's edge, but
Pages no longer associates `docs.ethlambda.xyz` with this repo, so it
404s.

## Fix

Add `cname: docs.ethlambda.xyz` to the deploy step so the `CNAME` file
is re-written on every deploy, making the custom domain binding durable.

## Note (manual step to restore *now*)

This makes future deploys self-healing. To restore the live site
immediately, the domain must also be re-set once: merge this (the deploy
on `main` will write the CNAME and re-bind the domain), or re-enter the
domain under **Settings → Pages → Custom domain**.

Author: MegaRedHand

.github/workflows/pr-main_mdbook.yml

@@ -110,3 +110,4 @@ jobs:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: book/html
           destination_dir: .
+          cname: docs.ethlambda.xyz