Add preview docs.

[Hyperkid123]

Short description on how to access preview environment before the /preview URL is decomisioned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.67%. Comparing base (4349edb) to head (7228c91).

:exclamation: Current head 7228c91 differs from pull request most recent head 7697151

Please upload reports for the commit 7697151 to get more accurate results.

Additional details and impacted files

[](https://app.codecov.io/gh/RedHatInsights/insights-chrome/pull/2882?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=RedHatInsights) ```diff @@ Coverage Diff @@ ## master #2882 +/- ## ========================================== + Coverage 63.59% 63.67% +0.08% ========================================== Files 202 202 Lines 4650 4650 Branches 858 858 ========================================== + Hits 2957 2961 +4 + Misses 1683 1679 -4 Partials 10 10 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/RedHatInsights/insights-chrome/pull/2882/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=RedHatInsights)

[fhlavac]

/retest