switch index to ocs

[tobiasKaminsky]

Fix #828

• webfrontend generates url with "index.php" and thus does not work.

[ChristophWurst]

Dear @tobiasKaminsky,
Thank you very much for the contribution.

This seems to convert the internal API of the app to a public API. To meet the standards of a public API, there has to be documentation about it in the developer documentation of Nextcloud. There are two options for an OCS documentation

1. Manually document the endpoints.
2. Use the OpenAPI generator to auto-generate a machine-readable metadata file.

Please note that once the API is public, it can no longer change in breaking ways for one year.

[ChristophWurst]

Please also see #458. I think the two PRs have the same goal.