chore(iOS, SplitView): Add internal documentation for SplitView

[t0maboro]

Description

This PR covers current SplitView APIs with a documentation in Doxygen format.

Closes: https://github.com/software-mansion/react-native-screens-labs/issues/289

Changes

• Covered all native SplitView classes with documentation

Test code and steps to reproduce

No functional changes

Checklist

• Included code example that can be used to test this change
• Ensured that CI passes

[kkafar]

I haven't read every single comment, just took a quick look & added some remarks.

General remark is that it is not necessary to have a doc comment for every single method / property, especially in cases where the comment does not give any additional context about method semantics / behaviour above repeating return value type & prop name.

My suggestion would be to comment only where there are some internal invariants or behaviours / interactions that are not contained directly in the prop / method name.

This is in large part a matter of taste, therefore I won't contest this that hard.

Below I added some remarks to indicate what I consider useful and what not, just to maybe give you some guidance.

[kkafar]

E.g. this doc comment is useful & nice that we have it as it brings some additional context not directly inferable from the prop type & name.