Bug 28189: Move swagger file to YAML format

Bug 28189: Move swagger file to YAML format

This patch changes the base OpenAPI file (swagger.json) into YAML. The
motivation for this, is adding more documentation, in Markdown.

JSON doesn't accept multiline strings, so this seems like a good move
for readability.

To test:
1. Verify your API is functional
2. Apply this patch
3. Repeat 1
=> SUCCESS: No changes, really
4. Point your browser to /api/v1/.html
=> SUCCESS: Some nicely formatted description of the API can be seen. It
includes information about x-koha-library.
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>