Bug 28020: (follow-up) Add docs about error_code on the API

Bug 28020: (follow-up) Add docs about error_code on the API

This patch intends to be a guide for inserting Markdown documentation
for error codes. The idea is that it can be copied and pasted as-is in
new routes. And adapted to new error codes.

To test:
1. Apply this patch
2. Run:
   $ npx redoc-cli@0.10.4 bundle --cdn --output index.html \
            api/v1/swagger/swagger.yaml
=> SUCCESS: It builds correctly
3. Open index.tml on your browser
4. Pick a route, and see the 500 status description includes information
   about the possible `error_code` values.
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>