Add link icon and period guidance to Polaris

[evija]

Feature request summary

Add the link icon to Polaris, and give guidance on where to add the period.

The period should be after the link icon, and it shouldn't be part of the link itself.

Rationale

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Consistency across Shopify. This icon shows up in a lot of places in Shopify, and it's currently treated differently.

This is how it can look today - the period can be found before the icon, after, or it's completely omitted.

This is how the current Link guidelines look like:

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[Jen-Bader]

Additional considerations: (clarified/expanded June 2, 2021)

1. Please specify w/in Polaris when the icon is used. I’ve heard (tho haven’t seen it documented) that we use the icon when when link sends the merchant outside of the app/product they’re using, e.g., if we send them to help docs. Update: a colleague surfaced this info in the Link component page in Polaris. Awesome! I'd looked here but missed it, as I was skimming for the icon itself and not "external links" language.

2. For your consideration re: where to put icons related to periods: my perspective is that icons do not belong inside of a sentence/before the period unless the sentence is about the icon itself.
   Reasons:

• I do not regard icons as part of a sentence structure (unless the sentence is about the icon itself and includes it as a reference)
• the shape of the external link means that it appears there is an extra space between the icon and the period when the period appears after the icon, which draws unnecessary attention to it b/c it looks like an error.

Including icons outside of the sentence means the non-linked period falls between linked text and linked icon, which is disruptive. Potential approach: omit periods if an external link icon is included. Reference the "Bills" example in the last image in this post.

This idea needs to be vetted b/c it then calls into question the rule to use periods for complete sentences ending in a link when an external link is not included.

3. In the case of "learn more" links such as the one used in the feature request summary above
   ,
   I recommend introducing language that is explicit about eliminating “about [topic]” in all cases where context makes the topic clear. Reason: the Polaris visual for "Links in a sentence" section might be construed as promoting the idea of specifying the topic, which isn't necessary if the context makes it clear.
   

Omitting the period after "Learn more" as a standalone, e.g., “Learn moreabout Google Analytics.,
is supported by Polaris as shown in the below image:

4. Also needed: documentation on using 2 links w/in the same sentence, as shown in the last scenario of the below image:
   

Lastly, thanks for the awesome work you all do!

[jjgali]

closing in favor of #5382