Rewrites to PlantUML page

What does this MR do?

#34082 (closed) was created to request some fixes to the PlantUML integration page at https://docs.gitlab.com/ee/administration/integration/plantuml.html. While there, I realized I should do a larger cleanup, because the page needed some extra attention.

I'm cc'ing @danielgruesso because this page falls under devopscreate groupsource code. Daniel, if you want to focus on the first commit you can confirm it handles the problem identified in #34082 (closed). Also, look for any facts I might've misplaced. Feel free to have opinions on style, but don't feel compelled to dig in on style issues; I've got @cnorris for that.

1. Added a numbered procedure at the top of the page that points to the subheadings, which are the major steps to take in integrating PlantUML with GitLab.
2. Re-flowed text into paragraphs containing multiple sentences, when possible.
3. Put procedures into ordered lists, to make clear they are a list of steps.
4. Rearranged the order of the page. I put the explanation of how to use the diagrams near the top of the page, because I think it's the real reason most people will visit this page. The installation instructions can sit lower down, because they're cross-linked from the top of the page.
5. A lot of rewriting for active voice, brevity, and tone.

Related issues

Closes #34082 (closed)

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