Clarify PR Merging Rules in Contributing Guide

[TrialDragon]

Objective

Centralize, and add back missing, PR merging rules to the Contributing Guide.

Solution

Put the four merging rules in the Reviewing Pull Requests page under How Pull Requests are merged. Deduplicate partial mentions of these rules, and instead direct to this section as a single source of truth.

Testing

Run zola serve --open on the trialdragon/clarify_pr_merge_rules branch, and check that the changed areas render correctly, and that Zola does indeed run.

[alice-i-cecile]

@lewiszlw could you take a look at this one too? :)

[lewiszlw]

The content looks good to me. But I didn't find the original text of How Pull Requests are merged in main repo.

[TrialDragon]

The content looks good to me. But I didn't find the original text of How Pull Requests are merged in main repo.

The original text was in docs/the_bevy_organization.md under the Maintainer header. I moved it to Reviewing Pull Requests here since it is useful info for plenty of contributors, and could get lost or forgotten in its general original place.

[mweatherley]

Just some grammar nitpicks. Looks good otherwise.