Improve message generation tutorial and example

[mjcarroll]

This expands the example, and adds some more documentation about how message generation works in a tutorial.

While there is some duplication between them, I think there is value in having the tutorial, as those are what gets indexed in the documentation as well as coverage in the tutorial party. I think there is value in having the example as that gets CI coverage.

[codecov]

Codecov Report

Merging #386 (b6ece1e) into gz-msgs10 (a6fbd92) will not change coverage.
The diff coverage is n/a.

❗ Current head b6ece1e differs from pull request most recent head 2c79db3. Consider uploading reports for the commit 2c79db3 to get more accurate results

@@            Coverage Diff             @@
##           gz-msgs10     #386   +/-   ##
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  Coverage      97.26%   97.26%           
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  Files             27       27           
  Lines           1170     1170           
==========================================
  Hits            1138     1138           
  Misses            32       32           

[azeey]

I've added the new file to tutorials.md.in and made some changes so that images are rendered properly in 2c79db3

[azeey]

LGTM!