Documentation: convert path-lookup from markdown to resturctured text

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>