[Feature]: Outline level support in front matter config

[hustcer]

What problem does this feature solve?

Currently the outline in front matter config can be set to false to disable it, and if it was enabled the outline will be generated automatically like this one: https://rspress.dev/guide/start/introduction.html The outline was extracted from h2/h3/h4 and it satisfy most use cases. However, in some rare cases I just need only one level outline extracted from h2 that's:
from:

• L1.0
  • L1.0.1
  • L1.0.2
    • L1.0.2.1
• L1.1
• L1.2

to:

• L1.0
• L1.1
• L1.2

What does the proposed API look like?

The front matter config of outline could be set to false, 1, 2, 3 or add another outlineLevel config which accept 1, 2, 3