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

从 material.angular.io/guide/theming 中受影响的主题指南开始，并比较其中的调色板和强调色示例。记录 A200、A100 和 A400 色调，解释为什么要单独声明强调色值，并澄清报告中提到的颜色映射和 Sass 自定义属性行为；该指南应在不存在所报告歧义的情况下回答这些问题。

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