Update Unity.gitignore - Added ignore for JetBrains Rider

[MickL]

Reasons for making this change:
Rider from JetBrains is a commonly used C# IDE. It will add a .idea folder which should be ignored in git.

[thatrobotdev]

See: #3529 (comment)

[github-actions]

This PR is stale because there have been no updates in 90 days. It will close after 180 days of inactivity. Leave a comment if you want to keep it open 😄

[MickL]

Yeah lets close unresolved issues, thats how to solve things the right way!

[thorrsson]

Hey @MickL
In case you missed the announcement, we added Stalebot to the repo to help prune down the hundreds of PRs that had been left unattended for as long as they had been.

As noted in the post, just adding a comment to the PR to identify it remains relevant is all you need to do to get it on our backlog.

[thorrsson]

I'd point out the Contribution Guidelines https://github.com/github/gitignore?tab=readme-ov-file#contributing-guidelines (We do have a rewrite of this coming but the guidance for Tool specific exclusions in a Language file will likely remain the same)

Specifically

Please consider the scope of your change. If your change is specific to a certain language or framework, then make sure the change is made to the template for that language or framework, rather than to the template for an editor, tool, or operating system.

Since the .idea folder isn't specific to Unity, nor Unity development I don't think this change belongs in this file. The .idea directory also stores Shared Run Configurations and some other artifacts that folks benefit from including in their committed changes. Ignoring this entire directory shouldn't be the standard, but optional for teams who choose not to include it.

** I acknowledge the .vs line in this file as well... it should not have been pushed through in the first place if we were to apply the Contribution guidelines **