restructure intro

- remove examples for simple object and collection object
- extra section for collections (to be specified)
- remove collection meta properties (they are not as cross-cutting as
the other meta properties, they are quite normal properties)

[ci skip]
basic collection specification
move collections into own group

add some example requests and responses
chose examples matching the response
fix cross ref

[ci skip]
improve wording

[ci skip]
collection-type: "collection" + more clarity

- more concise example-URLS
- explicitly mention "elements" property

[ci skip]
prepend /api/v3

- fewer possibilities to read the URL wrong (op.org/work_packages/1 is
already taken...)
- otherwise it would later become a pain to specify /api/v3 ^^

[ci skip]
equal naming for offset and cursor

[ci skip]
move pagination intro above tables

give some context for words used in the table, real explanation is kept
below the tables

[ci skip]
split that line

[ci skip]
update example links
Merge pull request #2164 from opf/feature/specify_collections