tep-1012: docs for TEP 1012

Basic documentation for custom timestamps

Signed-off-by: 35V LG84 <35vlg84-x4e6b92@e257.fi>
3 jobs for tep/tep-1012-custom-ts in 10 minutes and 39 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Test
passed #919630678
codecov

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99.81%
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test·jdk11

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passed #919630676
test·jdk8

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