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
 ----------
-leaf 
+leaf
 	An end node with data. This has a copy of the relevant key, along
 	with 'hlist' with routing table entries sorted by prefix length.
 	See struct leaf and struct leaf_info.
@@ -13,7 +17,7 @@ trie node or tnode
 
 A few concepts explained
 ------------------------
-Bits (tnode) 
+Bits (tnode)
 	The number of bits in the key segment used for indexing into the
 	child array - the "child index". See Level Compression.
 
@@ -23,7 +27,7 @@ Pos (tnode)
 
 Path Compression / skipped bits
 	Any given tnode is linked to from the child array of its parent, using
-	a segment of the key specified by the parent's "pos" and "bits" 
+	a segment of the key specified by the parent's "pos" and "bits"
 	In certain cases, this tnode's own "pos" will not be immediately
 	adjacent to the parent (pos+bits), but there will be some bits
 	in the key skipped over because they represent a single path with no
@@ -56,8 +60,8 @@ full_children
 Comments
 ---------
 
-We have tried to keep the structure of the code as close to fib_hash as 
-possible to allow verification and help up reviewing. 
+We have tried to keep the structure of the code as close to fib_hash as
+possible to allow verification and help up reviewing.
 
 fib_find_node()
 	A good start for understanding this code. This function implements a

Documentation/networking/index.rst

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