Update resource issue template

[machikoyasuda]

closes #307

• changes approval section to ask for approval in Slack
• adds a11y instructions link
• re-orders the questions to be in the same order as how the markdown file asks for them (date, title, asset link, category, tags) -- makes it easier to code the file

I forked the repo and tested out this issue template YML on my fork here: https://github.com/machikoyasuda/calitp.org/issues/new?assignees=&labels=content%2Cresources&projects=&template=new_resource.yml&title=New+resource+request

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[thekaveman]

Nice!