chore: add base PR template

[zivkovicmilos]

Description

This PR adds a base PR template for future contributions, that should be discussed in detail by the team.
It streamlines the PR review process by providing relevant information in the PR at a glance.

The base template is relatively simple to fill out. For more detailed PR templates, users can utilize the detailed_pr_template.md, by appending the template=detailed_pr_template.md query parameter (GitHub guide).

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist (for contributors)

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Additional comments

Tagging @moul and @harry-hov for visibility, as I can't add you as reviewers yet on the PR directly.

[zivkovicmilos]

@moul @harry-hov

I've updated the PR to include a simplified version of the PR template that shouldn't slow users down, but that also asks the user to provide more context for the changes.

The detailed PR template is still preserved, and users can access it using the template=detailed_pr_template.md query param on the new PR page (currently, GitHub doesn't provide a simple template select like with issues, so this is the only workaround).