infra: add MFM form for new post submission

[teauxfu]

• title (~50 characters)
• description/abstract: 2-3 sentences
• optional header image below title. keep phone resolution in mind. prefer short, wide photos. see DSA resources graphics folder
• author(s) to list? name or chapter?
• link to article text (MFM/Drive/attachment)
• when do you want it to be published? (at least one week out for editorial review)

[teauxfu]

the goal here is reducing friction when adding posts, so updating / improving the docs for cloning a post would help towards this as well

[teauxfu]

i'm not sure if this really will reduce friction, but in the meantime a github issue form has been added
https://github.com/dsa-ntc/brdsa.github.io/issues/new?template=post-submission.yml

i almost think we should just pull the post submission all the way up to the website? i dunno, maybe not worth the trouble