[docs]: autogen network documentation

# Documentation request

Cloud-init's network configuration is manually written[1][2]. Cloud-init recently added schema for validating network configuration[3][4], following the cloud-init's cloud-config validation model[5]. Cloud-config benefits from auto-generated documentation, which keeps documentation in sync with cloud-init's behavior. It would be nice if cloud-init's network documentation was also auto-generated from the schema.

[1] https://github.com/canonical/cloud-init/blob/main/doc/rtd/reference/network-config-format-v1.rst
[2] https://github.com/canonical/cloud-init/blob/main/doc/rtd/reference/network-config-format-v2.rst
[3] https://github.com/canonical/cloud-init/blob/main/cloudinit/config/schemas/schema-network-config-v1.json
[4] https://github.com/canonical/cloud-init/blob/main/cloudinit/config/schemas/schema-network-config-v2.json
[5] https://github.com/canonical/cloud-init/blob/main/cloudinit/config/schemas/schema-cloud-config-v1.json
[6] https://github.com/canonical/cloud-init/blob/main/doc/rtd/reference/modules.rst