doc: netlink: Change generated docs to limit TOC to depth 3

The tables of contents in the generated Netlink docs include individual
attribute definitions. This can make the contents exceedingly long and
repeats a lot of what is on the rest of the pages. See for example:

https://docs.kernel.org/networking/netlink_spec/tc.html

Add a depth limit to the contents directive in generated .rst files to
limit the contents depth to 3 levels. This reduces the contents to:

 - Family
   - Summary
   - Operations
     - op-one
     - op-two
     - ...
   - Definitions
     - struct-one
     - struct-two
     - enum-one
     - ...
   - Attribute sets
     - attrs-one
     - attrs-two
     - ...

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

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