docs: netlink: link to family documentations from spec info

To increase the chances of people finding the rendered docs
add a link to specs.rst and index.rst.

Add a label in the generated index.rst and while at it adjust
the title a little bit.

Reviewed-by: Breno Leitao <[e-mail]>
Reviewed-by: Donald Hunter <[e-mail]>
Reviewed-by: Jiri Pirko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/userspace-api/netlink/index.rst

···
    genetlink-legacy
    netlink-raw
 
-See also :ref:`Documentation/core-api/netlink.rst <kernel_netlink>`.
+See also:
+ - :ref:`Documentation/core-api/netlink.rst <kernel_netlink>`
+ - :ref:`Documentation/networking/netlink_spec/index.rst <specs>`

Documentation/userspace-api/netlink/specs.rst

···
 Internally kernel uses the YAML specs to generate:
 
  - the C uAPI header
- - documentation of the protocol as a ReST file
+ - documentation of the protocol as a ReST file - see :ref:`Documentation/networking/netlink_spec/index.rst <specs>`
  - policy tables for input attribute validation
  - operation tables
 

tools/net/ynl/ynl-gen-rst.py

···
     return "\n".join(lines)
 
 
+def rst_label(title: str) -> str:
+    """Return a formatted label"""
+    return f".. _{title}:\n\n"
+
+
 # Parsers
 # =======
 
···
     lines = []
 
     lines.append(rst_header())
-    lines.append(rst_title("Netlink Specification"))
+    lines.append(rst_label("specs"))
+    lines.append(rst_title("Netlink Family Specifications"))
     lines.append(rst_toctree(1))
 
     index_dir = os.path.dirname(output)