docs(contributing): add bash shell setup example for mise

[mrunalp]

Summary

Adds the missing Bash shell setup example for mise activate in the CONTRIBUTING guide. Fish and Zsh were already documented but Bash was not.

Related Issue

None, minor docs fix

Changes

Adds the missing Bash shell setup example for mise activate in the CONTRIBUTING guide. Fish and Zsh were already documented but Bash was not.

Testing

mise setup correctly in a bash environment

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[github-actions]

All contributors have signed the DCO ✍️ ✅
_{Posted by the DCO Assistant Lite bot.}

[mrunalp]

I have read the DCO document and I hereby sign the DCO.

[johntmyers]

recheck