extend init containers documentation

[RafalSkolasinski]

What this PR does / why we need it:

Initial documentation of Init Containers used for prepackaged model servers that dives into technical requirements that custom init container would need to satisfy.

Which issue(s) this PR fixes:

Closes #2055
Closes #2818

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[seldondev]

Tue Jan 5 21:49:51 UTC 2021
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=1

[seldondev]

Tue Jan 5 21:50:05 UTC 2021
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=2

[seldondev]

Tue Jan 5 21:52:01 UTC 2021
The logs for [lint] [4] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/4.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=4

[seldondev]

Tue Jan 5 21:52:10 UTC 2021
The logs for [pr-build] [3] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=3

[seldondev]

Tue Jan 5 21:54:59 UTC 2021
The logs for [pr-build] [5] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/5.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=5

[seldondev]

Tue Jan 5 21:55:17 UTC 2021
The logs for [lint] [6] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/6.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=6

[seldondev]

Thu Jan 7 18:56:52 UTC 2021
The logs for [pr-build] [7] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/7.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=7

[seldondev]

Thu Jan 7 18:57:04 UTC 2021
The logs for [lint] [8] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2814/8.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2814 --build=8

[axsaucedo]

/approve

[axsaucedo]

/approve

[seldondev]

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