Render the submodule headers as Markdown headers?

[asmeurer]

The release notes may look better if instead of formatting it like

• core
  • change 1
  • change 2
• solvers
  • change 1
  • change 2

* core
  * change 1
  * change 2
* solvers
  * change 1
  * change 2

we instead used Markdown headers, like

core

• change 1
• change 2

  solvers

• change 1
• change 2

#### core
* change 1
* change 2
#### solvers
* change 1
* change 2

It may also be easier for people to write release notes in this format, al they won't have to deal with nested bullets.

Finally, this would allow linking to the notes for a specific submodule, since every Markdown header gets an anchor.

Level 4 headers may be better level 3. Level 4 renders the same as just bold text, but with an anchor.

[asmeurer]

Although as for people writing the notes like this in the PR template, they would probably get confused by the header level, and write notes like

core

• change 1

  solvers

• change 2

So that may not be a great idea.