Notify Slack on successful Pages deploy

[ultrafellen]

Summary

Adds a Notify Slack step to the deploy job in deploy.yml. After a successful GitHub Pages deploy (if: success()), it posts a deploy log to Slack via chat.postMessage.

Message format (matches the team's existing convention):

[Docs] docs.hpp.io deployed successfully

```

https://github.com/hpp-io/docs/commit/

## How it works
- Credentials come from repo secrets **`SLACK_BOT_TOKEN`** + **`SLACK_CHANNEL_ID`** (already set on this repo).
- The commit message is passed via an **env var** (not inline `${{ }}`) and escaped with **`jq --arg`** → no script injection, safe with backticks/quotes/newlines.
- The step **fails the build** (`jq -e '.ok'`) if Slack returns an error (`not_in_channel`, `invalid_auth`, …) — no silent failures.
- Success-only (no failure notifications, per request).

## Prerequisites (already handled / to confirm)
- [x] Repo secrets `SLACK_BOT_TOKEN`, `SLACK_CHANNEL_ID` set.
- [x] Token validated via `auth.test` → bot `hpp_ops_bot` (workspace HPP/Aergo).
- [ ] Bot must have **`chat:write`** scope and be **invited to the channel** (couldn't verify non-destructively — the same bot already posts hub deploys to this channel, so this is expected to be in place).

## Test Plan
- [x] `deploy.yml` validates as YAML; `deploy` job steps = [Deploy to GitHub Pages, Notify Slack].
- [x] `jq` payload dry-run produces valid JSON and the expected Slack text.
- [ ] First real run: merge to `main` → Pages deploy → confirm the message lands in the channel (a misconfig will show as a red deploy).

> Note: this step only runs on push to `main`, so the Slack message is exercised on the next merge/deploy, not on this PR's CI.

[hayarobi]

혹시 어느 슬랙 채널로 알림이 오는지 알 수 있을까요?

[ultrafellen]

#dev_logs 로 설정해뒀습니다.

[hayarobi]

LGTM