Reconcile and consolidate type issue troubleshooting pages

[MaryaBelanger]

Fixing type promotion failures (non-promotion-reasons) and Fixing common type problems (sound-problems)

In terms of purpose and subject matter, I can't really tell if they're trying to address different topics. Seems like basically the same thing to me. Does anyone know what the goal was for each page originally?

@sfshaza2 you have a lot of commits early on for Fixing common type problems, any insight?

It seems like Fixing type promotion failures might've been created separately because of how it interacts with analyzer tooling, is that accurate @stereotype441 or @bwilkerson (you both left feedback on the first commit)

Just trying to understand the goal of each so I know what information to put where going forward, thanks!

---

Notes:

• I think /sound-problems could move to live under/alongside "The Dart type system" page
• and /non-promotion could be coupled with "Understanding null safety"