[libc][docs] Add net/if.h documentation by referring to POSIX standards (#122406)

This pull request introduces the following changes to the project with
reference to issue ( #122006 ):

1. **Documentation Update**:
- Added a new YAML file `if.yaml` under `net` to document network
interface functions and macros.
   - The `if.yaml` file includes the following functions and macros:
     - Functions:
       - `if_freenameindex`
       - `if_indextoname`
       - `if_nameindex`
       - `if_nametoindex`
     - Macros:
       - `IF_NAMESIZE`

2. **CMake Configuration Update**:
- Updated the `CMakeLists.txt` file to create the necessary directory
for the `net` headers.
- Included the `net/if` documentation in the Sphinx build configuration.

3. **Index Update**:
- Updated the `index.rst` file to include a reference to the newly added
`net/if` documentation.

**Purpose**:
- This pull request adds documentation for network interface functions
and macros, ensuring they are included in the project's documentation.
- Updates the CMake configuration to support the new documentation.

**Testing**:
- Verified that the new YAML file is correctly referenced in the
`index.rst`.
- Ensured that the documentation builds without errors and includes the
new network interface documentation.

Co-authored-by: Nick Desaulniers <ndesaulniers@google.com>