The typography theming docs previously had a link to typography-theming but that file doesn't exist. This removes the extension section since the only extension file (typography-theming) doesn't exist. Additionally some formatting that results in the headers not being rendered correctly in some instances.
### Context
In working on #5519 I tried to import "MaterialComponents/schemes/ColorScheme" and "MaterialComponents/schemes/TypographyScheme". Neither of these pods exist and you would get an error.
```bash
[!] CocoaPods could not find compatible versions for pod "MaterialComponents/schemes/ColorScheme":
In Podfile:
MaterialComponents/schemes/ColorScheme
None of your spec sources contain a spec satisfying the dependency: `MaterialComponents/schemes/ColorScheme`.
You have either:
* out-of-date source repos which you can update with `pod repo update` or with `pod install --repo-update`.
* mistyped the name or version.
* not added the source repo that hosts the Podspec to your Podfile.
Note: as of CocoaPods 1.0, `pod repo update` does not happen on `pod install` by default.
```
### The problem
We are adding an extra "scheme" at the end of the podname.
### The fix
Remove extra "scheme" at the end of the podname.
### Test
1. Create a fake project
2. Set your `Podfile` to:
```bash
pod 'MaterialComponentsAlpha',
:git => 'https://github.com/codeman7/material-components-ios.git',
:branch => 'schemes'
```
