slack bridge: Update doc for the new webhook based Slack Bridge.

This commit updates the Slack Bridge doc, primarily
guiding the user to use our Slack Webhook integration
and reformating the old instructions to a separate
legacy section.

Author: PieterCK

zulip/integrations/bridge_with_slack/README.md

@@ -4,6 +4,50 @@ This is a bridge between Slack and Zulip.
 
 ## Usage
 
+### 1. Zulip endpoint
+
+1. Create a generic Zulip bot, with a full name like `Slack Bot`.
+
+2. (Important) Subscribe the bot user to the Zulip stream you'd like to bridge your Slack
+   channel into.
+
+3. Create a [Slack Webhook integration bot][slack_webhook_doc] to get messages form Slack to Zulip.
+
+4. In the `zulip` section of the configuration file, enter the **Generic bot's** `zuliprc`
+   details (`email`, `api_key`, and `site`).
+
+5. After that, fill `integration_bot_email` with **Integration bot**'s email. Note that this is
+   the bot you created in step 3 and not in step 1.
+
+6. In the same section, also enter the Zulip `stream` and `topic`.
+
+7. (Important) In the `zulip` section of the configuration file, enter the 
+
+8. In the same section, also enter the Zulip `channel` and `topic`. Make sure
+   these match the ones your bot is sending to.
+
+### 2. Slack endpoint
+
+1. Go to the [Slack Apps menu][slack_apps_menu] and open the same Slack app that you have use
+   to set up the Slack Webhook integration in the previous section.
+
+2. Navigate to the "OAuth & Permissions" menu and note the "Bot User OAuth Token".
+   It starts with "xoxp-...".
+
+3. Scroll down to the "Scopes" section in the same page and make sure:
+   - "Bot Token Scopes" includes: `chat:write`
+   - "User Tokens Scopes" includes: `chat:write`
+
+4. In the `slack` section of the Zulip-Slack bridge configuration file, enter the bot name
+   (e.g "zulip_mirror"), token (e.g xoxp-...), and the channel ID (note: must be ID, not name).
+
+### Running the bridge
+
+Run Slack Bridge: `python3 run-slack-bridge`
+
+<details>
+<summary>Legacy: If you're still using classic Slack app, follow this instruction instead!</summary>
+
 ### 1. Zulip endpoint
 1. Create a generic Zulip bot, with a full name like `Slack Bot`.
 2. (Important) Subscribe the bot user to the Zulip stream you'd like to bridge your Slack
@@ -31,4 +75,12 @@ This is a bridge between Slack and Zulip.
 
 ### Running the bridge
 
-Run `python3 run-slack-bridge`
+Run Legacy Slack Bridge: `python3 run-slack-bridge --legacy`
+
+</details>
+
+[incoming_webhook_doc] = https://zulip.com/help/add-a-bot-or-integration#add-a-bot-or-integration
+
+[slack_webhook_doc] = https://zulip.com/integrations/doc/slack
+
+[slack_apps_menu] = https://api.slack.com/apps