[libc][docs] Add netinet/in header documentation by referring to POSIX standards (#122411)

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

1. **Documentation Update**:
- Added a new YAML file `in.yaml` to document network protocol and
address macros.
   - The `in.yaml` file includes the following macros:
     - `IPPROTO_IP`
     - `IPPROTO_IPV6`
     - `IPPROTO_ICMP`
     - `IPPROTO_RAW`
     - `IPPROTO_TCP`
     - `IPPROTO_UDP`
     - `INADDR_ANY`
     - `INADDR_BROADCAST`
     - `INET_ADDRSTRLEN`
     - `IPV6_JOIN_GROUP`
     - `IPV6_LEAVE_GROUP`
     - `IPV6_MULTICAST_HOPS`
     - `IPV6_MULTICAST_IF`
     - `IPV6_MULTICAST_LOOP`
     - `IPV6_UNICAST_HOPS`
     - `IPV6_V6ONLY`
     - `IN6_IS_ADDR_UNSPECIFIED`
     - `IN6_IS_ADDR_LOOPBACK`
     - `IN6_IS_ADDR_MULTICAST`
     - `IN6_IS_ADDR_LINKLOCAL`
     - `IN6_IS_ADDR_SITELOCAL`
     - `IN6_IS_ADDR_V4MAPPED`
     - `IN6_IS_ADDR_V4COMPAT`
     - `IN6_IS_ADDR_MC_NODELOCAL`
     - `IN6_IS_ADDR_MC_LINKLOCAL`
     - `IN6_IS_ADDR_MC_SITELOCAL`
     - `IN6_IS_ADDR_MC_ORGLOCAL`
     - `IN6_IS_ADDR_MC_GLOBAL`

_I believe, all these macros are necessary and should be documented._

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

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

**Purpose**:
- This pull request adds documentation for network protocol and address
macros in the `netinet/in` header.
- 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.

This pull request ensures that the `netinet/in` header macros are
documented and included in the project's documentation, and updates the
CMake configuration to support these changes.

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