[Docs] Document how to become collaborator on the Arrow project

[raulcd]

Describe the enhancement requested

With the migration to GitHub we have started using the collaborator role which allows us to give triage role to some users.
I was myself a collaborator before being a committer. This was done in order to be able to label issues, assign them, etcetera.

The role is documented here from the INFRA wiki: https://cwiki.apache.org/confluence/display/INFRA/Git+-+.asf.yaml+features#Git.asf.yamlfeatures-AssigningexternalcollaboratorswiththetriageroleonGitHub

We should document how are we giving triage role so there's a clear way of giving the role to new users.

Component(s)

Documentation

[raulcd]

We have been adding collaborators and the process for being added as a collaborator is not clear. The role is useful in order to be able to triage for labeling and assigning issues.
At the moment the process is that someone proposes new collaborators on a PR and this usually get approved and merged by a committer. I would propose adding something like the following, not sure if we should do that here: https://github.com/apache/arrow-site/blob/main/committers.md as this is only for the apache/arrow repo:

### Becoming a collaborator

The collaborator role allows users to be given triage role to be able to help with triaging issues. The role allows users to label and assign issues.
A user can ask to become a collaborator or can be proposed by another community member when they have been collaborating in the project for a period of time. Collaborations can be but are not limited to: creating Pull requests, answering questions, creating issues, reviewing PRs, etcetera.
In order to propose someone as a collaborator you must create a PR adding the user to the collaborators list on `.asf.yaml`. Committers can review the past collaborations for the user and approve.

Collaborators that have been inactive for a period of time can be removed from the list of collaborators.

[raulcd]

@kou do you think this (or something like this) should be proposed to be added to the arrow-site repo or is there somewhere else you think this would fit better?

[kou]

I think that https://github.com/apache/arrow-site/blob/main/committers.md isn't suitable for collabrators because it focuses on "committer".
How about adding this to https://arrow.apache.org/docs/dev/developers/reviewing.html#labelling ? Labelling is one of important permissions for collaborators.