Issue title: [mdc-card] SASS mixin names potentially wrong in readme
 Issue body: 

## Bug report
The [mdc-card readme](https://github.com/material-components/material-components-web/tree/master/packages/mdc-card) indicates that there should be a sass mixin called `filled-color($color)`. There's no text indicating anything about a namespace. So, I try
```
 @use '@material/card/mdc-card';
div {
       @include fill-color(#DADCE0);
}
```

I get the error 

`
SassError: Undefined mixin.
`
pointing at the fill-color. But, sass mixin names are all over the place for MDC, so I know to try the following, just in case

`mdc-card-fill-color`

`card-fill-color`

Neither work. 

I'm looking through various files in the `mdc-card` node_module folder, but I don't see anything indicating that the mixin should be anything other than just `fill-color`. I have other mixins working on this repo, so it's not a general issue with SASS imports. 

### Your Environment:

<!-- please complete the following information -->

| Software         | Version(s) |
| ---------------- | ---------- |
| MDC Web      |      "@material/card": "^5.1.0",
  
| Browser          | chrome
| Operating System | Ubuntu 18.04



 Comments: 
 Comment 0: Refer to the [Sass module system](https://sass-lang.com/blog/the-module-system-is-launched) for information on how Sass modules work. The `@use` syntax defines the prefix. If no prefix is explicitly provided, it will default to the file name of what you're importing.

```scss
@use "@material/card/mdc-card"; // generates classes for .mdc-card
@use "@material/card"; // does not generate classes, is used to access mixins and variables

div {
  // The prefix for @use "@material/card" is `card`
  @include card.fill-color(#DADCE0);
}
```

Let me know if that helps!
 Comment 1: Thanks for the help, I had been reading the SASS documentation up and down and just wasn't grasping that. 
 Comment 2: I'd like to chime in and echo @komali2's concerns... the current SASS mixins documentation is deeply unhelpful across the board. Consider the [Buttons](https://material.io/develop/web/components/buttons/) page as an example -- as a relative beginner, I had no idea that this:
```scss
@use "@material/button/mdc-button";
```
is not how I was supposed to import `mdc-button` to use the Sass mixins. This is also the only example provided in the documentation, and the SASS mixins section doesn't mention anything about importing the component any differently than specified at the top of the page.

I found the solution to my problem by looking at this bug. That's not good developer UX.

/rant
 Comment 3: I actually still don't understand. 

I'm trying to use the mixin mentioned for mdc dialog. 

https://github.com/material-components/material-components-web/tree/master/packages/mdc-dialog#sass-mixins

It says there's a mixin called `title-ink-color($color, $opacity)`.

So this time I followed the pattern that we did together for button. 

```
 @use "@material/dialog/mdc-dialog";
 @use "@material/dialog";

```

I get the following error:


```
Module build failed (from ./node_modules/sass-loader/dist/cjs.js):
SassError: Can't find stylesheet to import.
    ╷
122 │ @use "@material/dialog";

```

Uh oh. So I try all the other tricks:

```
 @use "@material/dialog/mdc-dialog";
     @include title-ink-color(vars.$theme-primary, 1);
```

Nope. Error: `Undefined Mixin`. 

`     @include dialog-title-ink-color(vars.$theme-primary, 1);`

Nope. `undefined mixin`. 

```
     @include dialog.title-ink-color(vars.$theme-primary, 1);
```

Nope. `SassError: There is no module with the namespace "dialog".`

```
     @include mdc-dialog.title-ink-color(vars.$theme-primary, 1);
```
Nope. Undefined mixin.

```
     @include mdc-dialog-title-ink-color(vars.$theme-primary, 1);
```

Nope. Undefined mixin.

 Comment 4: I'll create a new issue, since this is closed
 Comment 5: @komali2 this is the correct way we are encouraging users to currently apply styles and use mixins.

```scss
@use "@material/dialog";

@include dialog.core-styles; // apply default styles

.my-dialog {
  // reference namespace from "@material/dialog". Defaults to "dialog"
  @include dialog.title-ink-color(blue, 1);
}
```
{'source': 'GithubIssues', 'repo_name_with_owner': 'material-components/material-components-web', 'issue_id': 579583716, 'repo_stars': 17713.0}