docs: replaced slack invite links with dub.sh link (fixes #10768)

[AnkitKrYadav]

• This change is worth documenting at https://docs.all-hands.dev/
• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

Summarize what the PR does, explaining any non-trivial design decisions.

This PR replaces the existing Slack invite link with https://dub.sh/openhands short link for improved accessibility and maintainability.

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Link of any specific issues this addresses:
Resolves #10768

[jamiechicago312]

I checked these files here (but did no run docs locally)
All slack invite links shown in these files have been replaced with https://dub.sh/openhands just as discussed in our slack thread to make it easy to replace expired slack links without PRs.
TY!

[mamoodi]

Thank you!