maint: Add stale github action

[MikeGoldsmith]

Description

Adds a GitHub Action to automatically mark stale PRs and close them after inactivity, adapted from the opentelemetry-java workflow.

• PRs with no activity for 14 days are marked stale by
  • by adding a stale comment from configuration
  • adding the "Stale" label
• Stale PRs with no activity for a further 14 days are closed and add the close comment
• PRs with an exempt label are ignored
• Added section to CONTRIBUTING.md
• Issues are not affected

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

CI-only change, no code tests applicable.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[tammy-baylis-swi]

Thank you @MikeGoldsmith for making this happen!

Please could you also whip up a quick, generalized explanation under CONTRIBUTING Pull Requests.

I think the 14+7 day rule is generally fair. But maybe a 14+14 would be safer?

EDIT: Shoot I'm on the wrong PR 😉 will go to the other one open-telemetry/opentelemetry-python-contrib#4220

[MikeGoldsmith]

Thank you @MikeGoldsmith for making this happen!

Please could you also whip up a quick, generalized explanation under CONTRIBUTING Pull Requests.

I think the 14+7 day rule is generally fair. But maybe a 14+14 would be safer?

EDIT: Shoot I'm on the wrong PR 😉 will go to the other one open-telemetry/opentelemetry-python-contrib#4220

I do want to add the GHA to both core and contrib, but happy to tweak there first then copy here.

[MikeGoldsmith]

I've updated this PR to match the one we have open for contrib:

• increase stale / close days to 14+14
• add exempt PR labels
• add stale PRs section to CONTRIBUTING.md