- documentation aligned (description in the attributes)
- added clarification about general license according to spdx and specific license

The former yml file was just a translation of the json schema. Now the
yml file describes the whole API. Information about pagination was
missing as well as HTTP error messages.

yml file had the wrong indention in arrays. Fixing this improved
readability.

There was a naming error calling the MOOChub schema "moochub_schema.json"
instead of "moochub-schema.json". This caused problems in matching the
correct files in the github repository.

There was a mistake that "instructor" was not an array. This was in
contradiction to the defined schema.

There were some spaces in the honorific prefix missing in some
descriptions and examples. This was a clear mistake and made
reviewing changes unnecessary harder.

There was a problem that with generating the yml file by a
script the quotation marks were sometimes missing in the yml.
This caused a lot of trouble in reviewing changes in the
file, since all missing quotation marks were treated as
changes.

The script was updated and the file shows quotation marks now
where needed.

The affiliation in the instructor attribute was wrongly
indented. It was part of the image attribute. By correcting
the indention the affiliation is now part of the Person
object in the instructor attribute as intended.

There were some problems with data types of attributes.
Namely, some attributes can have a value of a defined type
or can be 'null'. In yml this was represented as an array.
As it is in the json. For consistency, the yml will now
only show the data type in the type attribute and has a
nullable attribute as before.

There were still issues with quotation marks that showed
changes where are none.

In the older version, empty attributes contained the value
'null'. This will be the case also for the newer version.
This way, no unnecessary tracking of changes occurs.

A few discrepancies to the old file were detected and they
are only of cosmetic nature. These were fixed to allow for
better comparability of real changes in the yml file.

Another alignment to fit the od style to avoid
unnecessary change tracking.

The repeatFrequency contained the following
attributes which should be on the same level
as repeateFrequency. The mistake is corrected
to fit the intended schema.

This is also removing some validation errors.

The LearningItem object in the hasPart attribute had a
mistake. The required attribute was on the wrong level.
Correction of will make the attribute hasPart work as
expected again.

This also erases a validation error in the yml file.

There was a problem in the "Problem" part of the yml file.
the required attribute as well as readOnly and x-examples
were at the wrong level.

Fixing this erases validation errors in the yml file and
leads to fewer unwanted changes to be tracked.

Also corrects a mistake that tags now is an array as
intended.

The indention of the old file and the new version were
different. This made it impossible to track changes.

The level the data object was stored in the JSON schema was
wrong. Now the data is stored as intended at the same level
as the links.

The There was a problem that the license attribute contained
an array of arrays of License objects. Intended is to have
the license attribute of the course storing an array of License
objects.

The There was a problem that the educationalLevel attribute
did not contain an array of EducationalLevel objects. Intended
is to have the educationalLevel attribute of the course storing
an array of EducationalLevel objects.

The example is now matching the identifier and is pointing
at the respective spdx url. It complies to the rules set in
the description now.

Before the change the link pointed at images on the azure
server and are not valid any longer. The new links are
working and point an existing image that is reachable.

The changes could not be reviewed because of too many
changes. The referencing style is now again like in the
former version.

There still seems to be an advantage in outsourcing
repeating parts in extra files.

Only minor changes were made. The functionality
was not altered. The change is mainly for
solving inconveniences, update examples, ...