Added kroki.io to diagramming chapter

[larkly]

Thank you for taking the time to work on a PR for Awesome-Sysadmin!

To ensure your PR is dealt with swiftly please check the following:

• Your additions are Free software
• Software your are submitting is not your own, unless you have a healthy ecosystem with a few contributors (which aren't your sock puppet accounts).
• Submit one item per pull request. This eases reviewing and speeds up inclusion.
• Format your submission as follows, where Demo and Clients are optional.
  Do not add a duplicate Source code link if it is the same as the main link.
  Keep the short description under 80 characters and use sentence case
  for it, even if the project's webpage or readme uses another capitalisation.
  Demo links should only be used for interactive demos, i.e. not video demonstrations.
  - [Name](http://homepage/) - Short description, under 250 characters, sentence case. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `License` `Language`
• Additions are inserted preserving alphabetical order.
• Additions are not already listed at awesome-selfhosted
• The Language tag is the main server-side requirement for the software. Don't include frameworks or specific dialects.
• You have searched the repository for any relevant issues or PRs, including closed ones.
• Any category you are creating has the minimum requirement of 3 items.
• Any software project you are adding to the list is actively maintained.
• The pull request title is informative, unlike "Update README.md".
  Suggested titles: "Add aaa to bbb" for adding software aaa to section bbb,
  "Remove aaa from bbb" for removing, "Fix license for aaa", etc.

---

Please take some time to answer the following questions as best you can:

• Why is it awesome?

It's very useful for sysadmins to document with textual diagrams from i.e. Gitlab or other instances supporting Kroki. While the Kroki instance can be somewhere else, it's very useful to self host for sensitive diagrams.

• Have you used it? For how long?

I've used it for about a year or so.

• Is this in a personal or professional setup?

Professional and personal.

• How many devices/users/services/... do you manage with it?

Unknown, it's used by the people who create PlantUML or other types of diagrams in Gitlab. Personally I also use it in my static website blog (mkdocs supports Kroki)

• Biggest pros/cons compared to other solutions?

Except PlantUML there are no other solutions other than systems supporting this themselves.

• Any other comments about your use case, things you've found excellent, limitations you've encountered... ?

[xrat]

Quite impressed! I did not know about Kroki before, had a look just now, and I am thankful for the pull request. IMO, Kroki would be a worthwhile addition to the list.

I wonder whether it should follow #416, though, and link to yuzutech/kroki!?