Allow setting uneditable Markdown in issue forms

[Josh-Cena]

Related to #4599, but that one's going in the wrong direction by assuming that Markdown elements would be showing up in the submitted issue, which may cause this valid request go away unnoticed.

I wish there's a way to set uneditable Markdown which does appear in the submitted issue. This could be as simple as allowing contentEditable: false in the attributes field of textArea and input. The most obvious use-case is allowing me to set a title for the issue template.

[lfrancke]

I'd like this as well to include checklists for certain types of issues automatically.
This way whoever works on a ticket has a checklist to work on, ideally this'd even be hidden in the displayed form

[mnapoli]

Here is another use case we have: we want to communicate to users that:

• we prioritize feature requests based on community feedback
• users are encouraged to add 👍 reactions to the issue

So we want to add a fixed paragraph at the top of every issue, e.g.:

Please vote on this issue by adding a 👍 reaction to the original issue to help the community and maintainers prioritize this request.

Right now with issue form it doesn't look like it's possible unfortunately, is that correct?

[SirSpidey]

Pulling together related requests for renderable Markdown:https://github.com/orgs/community/discussions/63402#discussioncomment-10341167