BUG:add Discord invitation for contributions

[JerryWu1234]

why

Replace expiring Slack invitation URLs with permanent workspace link. Add Discord as additional community option.

what changed

• Updated Slack links from invitation URL to permanent workspace URL (https://stagehand-dev.slack.com) in header badge and Contributing section
• Added Discord link (https://discord.com/invite/stagehand) in Contributing section

test plan

• Verify Slack badge links to workspace URL
• Verify Contributing section shows both Slack and Discord links
• Click links to confirm accessibility
• Check README renders correctly on GitHub

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1362d9b

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[greptile-apps]

Greptile Overview

Greptile Summary

Updated Slack invitation URLs to permanent workspace links and added Discord as an additional community option.

• Replaced temporary Slack invitation URL with permanent workspace URL (stagehand-dev.slack.com) in header badge and Contributing section
• Added Discord community link in Contributing section
• Updated Quickstart Guide URL to correct versioned path (v3/first-steps/quickstart)

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Simple documentation update that replaces expiring URLs with permanent links and adds a new community option. No code changes, no logic changes, and straightforward improvements to user experience
• No files require special attention

Important Files Changed

File Analysis

Filename	Score	Overview
README.md	5/5	Updated Slack links from temporary invitation URLs to permanent workspace URL and added Discord community link

Sequence Diagram

sequenceDiagram
    participant User
    participant README
    participant Slack as Slack Workspace
    participant Discord as Discord Server
    
    Note over README: Documentation Update
    User->>README: View community links
    README-->>User: Display Slack workspace URL
    README-->>User: Display Discord invite URL
    
    alt User clicks Slack
        User->>Slack: Navigate to stagehand-dev.slack.com
        Slack-->>User: Show workspace login/signup
    else User clicks Discord
        User->>Discord: Navigate to discord.com/invite/stagehand
        Discord-->>User: Show Discord invite page
    end

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