docs: migrate hosting from GitHub Pages to IC asset canister

[marc0olo]

Closes #438

What this does

Moves docs hosting from dfinity.github.io/icp-cli/ to an IC asset canister at https://cli.internetcomputer.org.

Key changes:

• Removes /icp-cli/ base path prefix — URLs are now cli.icp.build/0.2/... instead of dfinity.github.io/icp-cli/0.2/...
• docs.yml now pushes built assets to docs-deployment branch instead of gh-pages
• New docs-deploy.yml workflow deploys docs-deployment to the IC asset canister on every push to that branch
• IC config files added: icp.yaml, .ic-assets.json5, .well-known/ic-domains

Already done (pre-merge):

• docs-deployment branch bootstrapped from gh-pages (path rewrites applied, IC config added)
• First deploy to ak73b-maaaa-aaaad-qlbgq-cai already triggered — verify at https://ak73b-maaaa-aaaad-qlbgq-cai.icp0.io

Before merging

• Verify https://ak73b-maaaa-aaaad-qlbgq-cai.icp0.io looks correct (all versions, version switcher, links)
• Confirm the IC mainnet GitHub environment exists with the DFX_IDENTITY_DESIGN_TEAM secret — the docs-deploy.yml workflow requires it
• DNS is configured separately (CNAME cli.internetcomputer.org → icp1.io)

After merging

The first push to main will rebuild and redeploy all content. gh-pages is not modified by this PR — leave it untouched until the new deployment is confirmed working. Once confirmed, replace gh-pages with redirect pages (/icp-cli/* → cli.icp.build/*) as described in #438.

⚠️ Do not merge until the canister URL is verified

The docs-deployment branch and canister already exist. Merging only changes what CI does going forward — it does not affect the already-deployed content.

[marc0olo]

Post-merge: rebuild old version docs

After merging, the /0.1/ and /0.2/ folders on docs-deployment still have cli.icp.build baked into their canonical URLs (from the bootstrap). They won't be rebuilt automatically by a push to main.

Fix by triggering a rebuild for each old version:

# Rebuild /0.1/
git push origin docs/v0.1

# Rebuild /0.2/ (branch doesn't exist yet, create from latest tag)
git checkout v0.2.1-beta.0  # or latest v0.2.x tag
git checkout -b docs/v0.2
git push origin docs/v0.2

Each push triggers docs.yml → rebuilds the version with cli.internetcomputer.org → deploys via docs-deploy.yml.