docs(theming): theming guide is missing documentation on the accent hues (A200, A100, etc)

### Documentation Feedback

In your documentation when you talk about "palette" you say "These identifier numbers include 50, and then each 100 value between 100 and 900" and you give an example.
50: #e8eaf6,
100: #c5cae9,
200: #9fa8da,
300: #7986cb,
// ... continues to 900
contrast: (
50: rgba(black, 0.87),
100: rgba(black, 0.87),
200: rgba(black, 0.87),
300: white,
// ... continues to 900
)
);

But later when you talk about theme when you define the primary color OK no problem because you are using a value consistent with your declaration in the palette ($my-primary: mat.define-palette(mat.$indigo-palette, 500)).
But for the accent color there is a problem because the values ​​no longer match your palette declaration (define-palette(mat.$pink-palette, A200, A100, A400)) -> what about A200, A100, A400 ?

Moreover why you need to declare in each palette "accent" values if in you theme you declare an accent color with all the hues from 50 to 900 ?

This problem has consequences in the code if you want to manage the light and dark theme with the "new" declaration model (with color in the map).
If you declare a palette without Axxx and try to inject it in the map at the 'color'/'background'
level you obtain "color: null is not a color" error.
Moreover you can't use SASS custom properties "var(--xxxxxx)" in the palette to manage foreground and bacground !!

### Affected documentation page

https://material.angular.io/guide/theming

Inizia dalla guida di theming interessata all'indirizzo material.angular.io/guide/theming e confronta i relativi esempi di palette e colori di accento. Documenta le tonalità A200, A100 e A400, spiega perché i valori di accento vengono dichiarati separatamente e chiarisci il comportamento segnalato delle mappe di colori e delle proprietà personalizzate Sass; la guida dovrebbe rispondere a queste domande senza l'ambiguità segnalata.

Scritto dal modello di indicizzazione a partire dal testo della issue.