Update issue and PR templates

[MakisH]

This PR updates the issue and PR templates to make them more concise and useful.

Anything else that I missed?

PR template

• Adds reminders to update the website content (tutorials overview and sidebar) and tutorial README files
• Adds resources, pointing to the contributing tutorials and system tests documentation.
• Removes the unactionable "I will remember to squash & merge"

Issue template

• Makes it shorter (to make it more inviting to read)
• Points directly to the forum
• Renames it to "bug", to better communicate the use case