docs: networking: convert fib_trie.txt to ReST

- add SPDX header;
- adjust title markup;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/fib_trie.txt → Documentation/networking/fib_trie.rst

-			LC-trie implementation notes.
+.. SPDX-License-Identifier: GPL-2.0
+
+============================
+LC-trie implementation notes
+============================
 
 Node types
 ----------

Documentation/networking/index.rst

@@ -55,6 +55,7 @@ Contents:
    dns_resolver
    driver
    eql
+   fib_trie
 
 .. only::  subproject and html