Add known issue about content packs in 0.17 series

[benjaoming]

Summary

This addresses #5561 so that people are finally informed that this is a flaw of 0.17.

We will prioritize getting 0.18 out ASAP, just for the sake of this content pack update!

This is a docs upgrade so it mainly just needs to be merged to master and 0.17.x to get exposure on RTD.

Additional note: I'm removing the 0.17.5 milestone.

TODO

If not all TODOs are marked, this PR is considered WIP (work in progress)

• Has documentation been written/updated?
• Have you written release notes for the upcoming release?

Reviewer guidance

n/a

Issues addressed

#5561

[codecov]

Codecov Report

Merging #5562 into 0.17.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           0.17.x    #5562   +/-   ##
=======================================
  Coverage   66.13%   66.13%           
=======================================
  Files         113      113           
  Lines        6506     6506           
=======================================
  Hits         4303     4303           
  Misses       2203     2203

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[benjaoming]

Merging this so it becomes visible to users, feel free to add comments hind-sight!