Add documentation file for new authorization model.

Write some explanation on the admin side of new authorization model.
Update contents.
Remove obsolete field from REST API docs.
15 jobs for auth-docs in 9 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #60573
amd64
dispatcher-debian-10

00:03:12

passed #60571
amd64
dispatcher-debian-9

00:03:18

passed #60574
amd64
server-debian-10

00:02:39

passed #60572
amd64
server-debian-9

00:03:07

 
  Analyze
passed #60576
amd64
black

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passed #60577
amd64
code_quality

00:00:23

passed #60578
amd64
dockerfiles

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passed #60579
amd64
job-schema

00:00:27

passed #60580
amd64
pylint

00:02:44

passed #60575
amd64-dind allowed to fail
sast

00:01:53

 
  Build
passed #60583
amd64
amd64/pkg-debian-10

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passed #60582
amd64
amd64/pkg-debian-9

00:01:41

passed #60581
amd64
doc

00:01:14

 
  Docker
passed #60584
amd64-dind
amd64/lava-dispatcher

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passed #60585
amd64-dind
amd64/lava-server

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