[armanist]

Brings three getting-started pages in line with the current starter project (quantum-php/project#204) and the serve command.

What changed

• getting-started/project-structure.md: remove the libraries/ folder, which the starter project no longer ships; point custom classes to shared/ (autoloaded as Shared\); describe cache/ (the file cache and the rate limiter store their files in cache/data, git ignores its contents); merge the duplicated .env.example bullet.
• installation.md: replace the PHP 7.4 note with a php -v check, fix a stray backslash and a missing bash fence, and give the default server address (http://127.0.0.1:8000/, next free port if 8000 is busy) with a link to the serve options.
• cli/development-server.md: state the default address and the next-free-port behavior, linking to serve.

Changed paths

• v3.0/getting-started/project-structure.md
• v3.0/installation.md
• v3.0/cli/development-server.md

SUMMARY.md: no change.