Repository Maintenance Improvements

[cderici]

Description

This adds a couple of upgrades/fixes for repository maintenance:

• Brings back the issue/feature request templates
• Adds a github action/stale bot to automatically decay the issues/PRs that are not updated for a long time. In the current configuration, every issue/PR that hasn't been updated for 30 days will be marked with an incomplete label, and will be closed with a nice message if it's still not updated for 5 days after that mark.
• Adds a document for contributing guidelines for outside contributions. The page will look like this: https://github.com/cderici/cderici.github.io/blob/master/docs/CONTRIBUTING.md

QA Steps

No QA needed.

[anvial]

btw, am I right that if we remove the incomplete label from issue that will drop the timer.

If not, I think that we should add to exempt labels branch labels (2.9/3.0/...)m because for us that means triage.

[cderici]

Just for future reference I'll note my answers here:

btw, am I right that if we remove the incomplete label from issue that will drop the timer.

This is correct.

If not, I think that we should add to exempt labels branch labels (2.9/3.0/...)m because for us that means triage.

Already added some exempt labels to express these cases (awaiting approval/work in progress) without worrying about the stale bot.

exempt-issue-labels: 'awaiting-approval,work-in-progress'
exempt-pr-labels: 'awaiting-approval,work-in-progress'

[cderici]

/merge