chore(github): Replace feature request issue template with ideas discussions template

[Savio-Sou]

Description

Problem*

Noir's feature requests are currently collected and managed as GitHub Issues.

This is not ideal as:

Suggesting "Issues" is unintuitive

The term "Issue" could carry the meaning of "problems / bugs", which leads to GitHub Issues not necessarily being the first place users would check if they would like to raise a suggestion.

Upvoting is unintuitive

The thumbs-up emoji can be used as a pseudo upvote, but it is hidden behind the "reaction" smiley face and doesn't show up in the Issue list unless explicitly sorting with it.

Mental barrier

Filing a GitHub Issue often comes with a mental barrier that it should be logically-detailed and well-written, which might deter users from creating a suggestion.

Summary*

GitHub Discussions could potentially be a better place for collecting and managing feature requests.

This PR:

• Replaces the existing feature request GitHub Issue template with the ideas GitHub Discussions template (content is identical)
• Adds an "Ideas" contact link in the list of actions on the "New Issue" page (for users that are used to / prefer creating feature requests through the creating GitHub Issues workflow)

Additional Context

For a glimpse of how this would look like in action, checkout the demo noir-lang org: https://github.com/noir-lang-test/noir/issues/new/choose

For ideas that contributors would like to work on, they can easily convert discussions into GitHub Issues and follow up with the usual GitHub engineering workflow:

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.