doc: update charter section on voting

[aduh95]

I've considered adding requirements regarding what tool to use, but after giving it some more thoughts, it's probably better if the choice of the tool is not chartered so if we need/want to change it, it doesn't require CPC approval. Instead, let's edit the charter so it forbids the use of a voting solution that would lack the features we deem important:

• The wording around how the "winner" is defined currently seems to assume there would be always two candidates. Although the current wording is not invalid for many-candidate votes depending how one define the word "vote", it makes more sense to write it in a way that assume there might be more than two candidates.
• Added a sentence that requires ballots not to be disclose before the vote closes. It's been a few years we've operated this way, IMO the upsides clearly outweigh the tradeoffs.

[aduh95]

Let me know when I need to bring this to the CPC..

There have been no objections and a few approvals from current TSC members, I think it's time to ask CPC to have a look on it.