[DOC] A doc page for the extract API

This doc page details the extract API and has the following structure:

- Service explanation.
- Expected successful flow
- Description of the 3 routes (request and response structure)
	- /parse
	- /get_results
	- /validate
- Hint for integration testing

closes odoo/odoo#61991

X-original-commit: af486cb8
Signed-off-by: Florian Daloze (fda) <fda@odoo.com>
Signed-off-by: Sébastien Mottet <smottet@users.noreply.github.com>