docs: pointing out breaking change for MatSidenav and MatDrawer in 7.0

[Isitar]

Added description to Breaking changes that the template for MatSidenav and MatDrawer have changed. Was done in this 0f83c18 commit.

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[Isitar]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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What to do if you already signed the CLA

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CLAs look good, thanks!

[crisbeto]

LGTM

[jelbourn]

I don't agree that this is a breaking change. Most of the time, we don't consider the structure of the component to constitute a formal public API. I do think it would be good to add a note to the changelog, but not in the "breaking changes" section. Additionally, the note should be more specific about what the change consists of rather than the broad statement that "the template has changed"

[Isitar]

It actually breaks the design of a page if you have a css selector like mat-select > ul for example.
Wheter it is a breaking change or not is up to you, for me it is important that it is noted somewhere and I don't have to investigate in the components why my upgraded site looks different.

[mmalerba]

@Isitar sorry I merged this before realizing the pr: merge ready label had been removed and had to revert, please re-create this PR

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