Add a section for etiquette when creating PRs, when multiple people wants to create the PR for the same issue

Describe the enhancement or feature you'd like
Sometimes more than one people want to create PR for the same issue/ticket. How do we handle this situation?

From my own experience since back in the days of the bpo/roundup, there seems to be a preference that when a patch already exists, we don't try to create a new patch but work to improve the existing patch.
I've seen core devs telling contributors to "let this person finish the patch". I think we should document this preference clearly so that new contributors are aware.

I believe that this is still a good common practise with GitHub PRs, to give courtesy to whoever claimed the ticket first, giving them time and opportunity to contribute and create a PR. For example, if someone opened an issue and expressed that they want to create the PR, then others should let them create the PR instead of writing their on PR in the meantime.

In this situations, sometimes, as triagers and core devs, we may need to remind other contributors gently, that "hey this ticket is already claimed by someone else, don't go and write your own PR."

However, in order to be fair and avoid people claiming tickets and not finishing it, (and therefore preventing the issue to be completed/preventing others from also contributing), we should specify a reasonable timeline for claiming/creating the PR.

Some suggestions (gathered from Discord):

• Document that, if someone else has claimed the PR, don't write your PR yet. Instead help them with it/answer questions/clarify the issue
• Ask for/provide the timeline for completing a PR.
  • Triagers/core devs can say "can you write the PR within X weeks?" (2 weeks as default, maybe depends on the complexity of the ticket?)
  • Person who claimed the PR can say, "I plan to create the PR within X weeks." (or other timeframe that works for them)
• After the initial "issue claim" timeline has passed, triager/core dev should check in on the progress, and ask if anything blocks them, and work towards helping/supporting the contributor in the PR creation.
• If after the specified time, they still have not started on the PR, at that time someone else can claim the ticket and create the PR.
• Emphasize that draft PRs are welcomed
• Emphasize that these are plans, which are allowed to change, and not firm commitment
• Emphasize that it is ok if they are no longer able to create a PR, and in fact preferred if they tell us, to give opportunities to others to contribute.

Additional context

This was discussed internally among a few triagers and core devs in the #triage Core Devs Discord channel.

I'm opening an issue so that we can discuss it further and potentially add such guidelines to the devguide/the new contributing guide.

审查现有的 devguide 和拟议的新 contributing guide，找到适合说明 contributor 行为规范的位置。为重叠的 claim、时间安排、Draft PR 以及释放 claim 制定指南，然后在记录之前与 triager 和核心开发者确认措辞和政策。