Documentation: convert path-lookup from markdown to resturctured text

This allows the document to be integrated with the main documentation
tree.
Changes include:
- rename from .md to .rst
- use `` for code, not single `
- use correct sub-section marking
- fix indented blocks, both code and non-code
- fix external-link markup

Signed-off-by: NeilBrown <neilb@suse.com>
[jc: changed the toctree organization a bit]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -359,3 +359,14 @@ encryption of files and directories.
     :maxdepth: 2
 
     fscrypt
+
+Pathname lookup
+===============
+
+Pathname lookup in Linux is a complex beast; the document linked below
+provides a comprehensive summary for those looking for the details.
+
+.. toctree::
+   :maxdepth: 2
+
+   path-lookup.rst