Add tertiary action guideline to modal section

[yangchen1]

Should be shipped with https://github.com/Shopify/polaris-styleguide/pull/2714

WHY are these changes introduced?

Following up on Polaris day explorations on tertiary guidelines in modals

WHAT is this pull request doing?

After:
Best practices section:

Content section:

[selenehinkley]

I remember seeing a slide deck with visual explorations. Are there screenshots available of do/don't examples we could add to the content section?

[BPScott]

No comments from me. Address Selene's requests then this is good to go.

[yangchen1]

Thanks so much for the comments @selenehinkley ! Will address on Friday

[selenehinkley]

I'm not sure why, but when I tophat the playground looks like this (and I can't see the added content):

Any idea's @BPScott? I'm following these instructions: https://github.com/Shopify/polaris-react/pull/1361/files

[selenehinkley]

@jonniesweb helped me look into the issue and we think it has to do with one of the added images

[yangchen1]

@selenehinkley Those images are added in the styleguide repo in a separate branch/PR, I'm so sorry I forgot to cross link it here

https://github.com/Shopify/polaris-styleguide/pull/2714

[selenehinkley]

Oh okay! So once that's merged then I 🎩 and it should show up, right?

[yangchen1]

ok @selenehinkley I just merged the other one, so that error should go away now! So sorry for blocking that

[danrosenthal]

Hey @yangchen1, is this good to go?

[yangchen1]

@BPScott Can you help with shipping this?

[danrosenthal]

Can you help with shipping this?

I've got you @yangchen1

[yangchen1]

Thank you @danrosenthal !!