Improve wording on notice preventing development from src directory.

[omarreiss]

Splitting this out of #43055 as a future, yet necessary iteration.

The wording on the notice we plan to serve to devs who are running WordPress develop from the src directory isn't very accessible or user friendly. We need to improve it and with it the documentation.

[netweb]

Related: #44228 "Read more about setting up your local development environment" needs to link somewhere useful

[arcturusink]

More instructions

[arcturusink]

I installed WordPress last week, and this page gave me a lot of trouble. I could not find many documentation on WordPress or information via Google Search on how to proceed from this page. Took around 2.5 hours to figure things out. I hope my input/experience is helpful for anyone who has to come across this page, and they have a better experience on knowing how to proceed. I know its not perfect, so any feedback is welcomed. I used <br> to break up the lines. Is there a more elegant way of doing it? I did not want to introduce html for the unordered list in the php code, so I hard-coded the dash.