docs(home): add discord invitation link

[sthuber90]

Description of your changes

Replace all occurrences of the Slack link with a link to the new Discord space. Add a Discord badge to the README.

How to verify this change

Review changes and build documentation locally to verify the new link is presented correctly.

Related issues, RFCs

Issue number: #1073

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

Breaking change checklist

N/A

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[dreamorosi]

Thank you for the PR!