chore: update governance document

[ematipico]

Summary

This PR update the governance document, to address the following:

• it clarifies the voting rules and dynamic by adding **private** to the word "thread"
• reduce the voting period from seven days to three. I have been noticing that seven days is a very long period, and the majority of voting sessions we've had finish even before three days
• it adds many clarifications around the expense policy and how leads can submit expenses. When I wrote this section, I took some knowledge for granted, which was a mistake on my end.

Test Plan

The majority of @biomejs/core-contributors must read and approve the PR

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