Update docs to clarify how to add content

[yakky]

Description

Describe:

Clarify how text content works by linking django CMS editor tutorial

References

Fix #636

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included (see docs)
• Usage documentation added in case of new features

[coveralls]

Coverage remained the same at 96.74% when pulling 576051f on docs/issue-636-content into 0e83441 on develop.

[codecov]

Codecov Report

Merging #642 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #642   +/-   ##
========================================
  Coverage    94.50%   94.50%           
========================================
  Files           22       22           
  Lines         1111     1111           
  Branches       138      138           
========================================
  Hits          1050     1050           
  Misses          23       23           
  Partials        38       38           

Flag	Coverage Δ
unittests	94.50% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e83441...576051f. Read the comment docs.