docs: the quickstart and README are out of date for the current setup and deploy flow

[armando-navarro]

The Quickstart (docs/install-and-setup.md) and the README have drifted from how a current Angular app is created and how AngularFire's ng add behaves, so a new user following them step by step hits several dead ends.

What I found

• Step 2 describes the old ng add. It says ng add @angular/fire adds Firebase config to environment variables and configures @NgModule for AngularFireModule. The current schematic writes provideFirebaseApp(...) into the app configuration with the config inlined, and creates no environment files.
• Steps 3 and 4 use file and class names the CLI no longer generates. They tell you to edit src/app/app.component.ts and class AppComponent. A current Angular CLI (both 20 and 21) scaffolds src/app/app.ts with class App.
• Step 6 tells you to run ng deploy, which fails on a fresh project. ng add no longer wires a deploy target, so ng deploy has nothing to run (related to ng deploy still doesn't work #3676).
• The newest Angular major needs a prerequisite that is not documented. On a brand-new Angular app, ng add @angular/fire can fail with a peer-dependency conflict because the default latest release trails the newest Angular major. The workaround, ng add @angular/fire@next, is not mentioned (related to @angular/cli v20. NPM cannot resolve @angular/fire #3667).
• Setup noise looks like an error. ng add can surface a punycode deprecation warning and a Firebase CLI prompt that are harmless but read as failures, with nothing in the docs to reassure a first-time user (related to DeprecationWarning: The 'punycode' module is deprecated when running ng add @angular/fire #3649).
• README rough edges. The "Example use" snippet has a brace typo and missing imports, a feature bullet still advertises the removed single-command deploy, the "developer preview" note misdescribes a library that has shipped stable majors for years, and the product-integrations table is unordered.

Scope

Documentation only. A pull request follows.