Improve new-snippet form UX

Spawned from !67859 (merged) where I discovered that the form to create a new snippet (https://gitlab.com/gitlab-org/gitlab/-/snippets/new) has some UX deficiencies:

• Fields don't have visible names so the documentation can reference them
• The only explanation for fields is in placeholder text. It disappears when the user starts typing.
• If the user adds more than one file, it's tough to distinguish the first file from the second and third, etc.
• I'm betting the screenreader accessibility could be improved.