125 edits later and down from 47096 to 38209 bytes, I'm kind of okay with mw:Gerrit/Tutorial. Purrfect is the enemy of good, I guess.

• Clarified between SSH Private key and SSH Public Key
• Replaced "extensions/examples" repository by "sandbox". This allows people to follow the steps without creating test patches for a production repository.
• Updated output of most commands shown as examples.
• Removed repository specific instructions (e.g. how to clone MW core or extensions or Vagrant issues) by linking to mw:Download_from_Git instead
• Made "Create a Wikimedia developer account" a separate step earlier in the list.
• Replaced <pre>, <source>, <code> with {{cmd}} with proper red highlighting of variables to replace, and green for commands. Still use <code> for file names and describing commands. Explained on top that you shall not enter the $ prefix of commands.
• Use one consistent username in instructions, instead of preilly, example, my_username, jpostlethwaite.
• Synced @example.com and @youremail.com addresses by using only @example.com.
• Removed editor-specific commands (vim) and distribution specific commands (apt-get).
• Mentioned Phabricator, as the example T1234 made no sense without any such context.
• Don't mix gerunds, imperatives, and "how to..." in section titles. Consistently use imperative.
• Changed level of "Other common situations" and "Editing via the web-interface" so they do NOT look anymore like part of the default steps under "Submit a patch".
• Removed numerous terminal output examples that were self-explaining. Easier to read.
• Converted "Linux & UNIX", "Windows" and "OS X" subsections into bullet points as they bloat the ToC. Renamed "OS X" to "Mac OS X".
• Removed numerous "alternative instructions". People need to follow one guide.
• Removed irrelevant historic info about Git and Gerrit - there is Wikipedia for that.
• Removed embedded videos which just duplicate text information. They are still linked under "See Also".

Potential future TODOs:

• Using bullet points vs using no bullet points in subsections is inconsistent
• "commit" vs "change" vs "diff" vs "patch" terminology is confusing and not really explained
• Not sure if "terminal" vs "command line" use is always correct (at least no "console" is used)
• Still wondering whether to indent all the {{TNT|Note}} items, because still so much to read sometimes which isn't relevant to getting stuff done.
• Examples under #Squash_several_commits_into_one_single_commit_via_rebase still puzzle me, but I don't know enough about that to fix it.
• #Editing_via_the_web-interface still welcomes cleanup. I have no idea if it refers to GWT or Polygerrit, but that is already covered in T227562.
• The whole https://www.mediawiki.org/wiki/Gerrit/Tutorial#How_code_is_reviewed_in_Gerrit section is confusing and partially duplicated with https://www.mediawiki.org/wiki/Gerrit/Code_review and https://www.mediawiki.org/wiki/Gerrit/Code_review/Getting_reviews