gh: Update issue templates

[SuaYoo]

Changes

• Adds issue type to each template
• Differentiates user-submitted "Change Request" from internal "Planned Feature". This allows us to separate user-submitted ideas from work we've planned through the new feature workflow, and automatically set the github project.
• Adds template for docs change
• Makes additional context section optional, I noticed many issues put "n/a" or similar in this section anyway.
• Disables blank issue adds generic "Task" issue template

[emma-sg]

I've found more or less the PR format you've been using useful too — how would you feel about making it a template here as well?