doc: net: Fix .rst rendering of net_cachelines pages

doc: net: Fix .rst rendering of net_cachelines pages

The doc pages under /networking/net_cachelines are unreadable because
they lack .rst formatting for the tabular text.

Add simple table markup and tidy up the table contents:

- remove dashes that represent empty cells because they render
  as bullets and are not needed
- replace 'struct_*' with 'struct *' in the first column so that
  sphinx can render links for any structs that appear in the docs

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241008165329.45647-1-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>