docs: update links to Material Design spec

[EdricChan03]

For color palettes, link to the archived spec since there isn't an equivalent in the 2018 revision.

[jelbourn]

LGTM

[ngbot]

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     missing required label: "target: *"
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     status "continuous-integration/travis-ci/pr" is pending

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[jelbourn]

Commit message fixup:

docs: update links to Material Design spec

For color palettes, link to the archived spec since there isn't an equivalent in the 2018 revision. 

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