[RNE Rewrite] docs(cpp): add missing C++ documentation in header files

[barhanc]

Description

Adds missing Doxygen documentation across C++ header files in packages/react-native-executorch/cpp/.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

N/A

Screenshots

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

[msluszniak]

Now when I read the names of the functions I'm wondering why we have such name convention. Str -> parseDtype, but reverse is named toString. For the next two the convention is unified, but we only know that we convert sth to scalar and we get sth from scalar. Maybe we should name it xFromY and xToY for both conversions? Especially, because we have dtypeToCvDepth and similar.

[barhanc]

Yeah, you are right. Let's wait with this PR until #1327 lands so we can change the names.