[docs] Debug environment variable suggestions

[mattstein]

The Issue

I didn’t get a chance to review

• Separate "build" from "up" stages in DDEV Start() #4919 before it merged and there are a few opportunities to improve the docs that came with it:

• It’s best to reiterate in every case, provided it doesn’t interrupt the reading flow, that commands are meant to be run. This is especially true when someone’s troubleshooting and we want them to be successful without knowing their technical comfort level with what’s going on.
• A callout was added with a heading that didn’t summarize the contents as much as label them, and it gave excessive emphasis to something that...
  • a) From its own language, is less useful to everyone and meant for DDEV developers.
  • b) Appeared ahead of, and thus took attention away from, another callout more broadly applicable to a beginner troubleshooting. (Stacking callouts is usually indicative of a flow problem with too much noise.)
  • c) Includes material for a different audience; existing troubleshooting steps seem careful and beginner-friendly, and this callout talks about a Dockerfile build process that abruptly assumes a different level of prerequisite knowledge—with no qualifying introduction or explanatory language.

How This PR Solves The Issue

This PR refines the newly-added docs and moves the callout into a subsection.

This doesn’t fix the audience-shift problem (c), but it moves the callout to a subheading in that first section, with a concise heading to describe what it covers. It also rewords the contents of the callout to topically introduce the environment variables with gentler context that’s hopefully just as accurate.

Manual Testing Instructions

Preview changes locally or inspect the automatic build:

• https://ddev--4949.org.readthedocs.build/en/4949/users/extend/customizing-images/#debugging-the-dockerfile-build
• https://ddev--4949.org.readthedocs.build/en/4949/users/usage/troubleshooting/#debug-environment-variables

Automated Testing Overview

n/a

Related Issue Link(s)

#4919

Release/Deployment Notes

n/a

[rfay]

Thanks so much for this. Looks reasonable to me.

[mattstein]

Please merge at whatever moment brings the most joy.