Adding info on iOS app extensions

[leighajarett]

Description of what this PR is changing or adding, and why:

This adds a new page that describes how Flutter developers can add iOS app extensions. This is a popular feature request: flutter/flutter#16092. It also adds a few plugins to the existing page that talks about Apple system libraries.

I am submitting this now to get the review started, but we shouldn't publish it until we have published the two codelabs (I am currently working on them and hoping to have a draft ready in the next few weeks) and this PR is merged: flutter/plugins#7114

I couldn't figure out how to get the images to be a bit smaller, or how to fix the numbered list with an image -- if anyone knows how to fix this that would be amazing!

Issues fixed by this PR (if any): #7448

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[domesticmouse]

the two codelabs (I am currently working on them and hoping to have a draft ready in the next few weeks)

Hey Leigha, can you add me to the codelab docs? Also, I'm curious if you plan to add the code for said codelabs to https://github.com/flutter/codelabs?

FYI @sfshaza2

[sfshaza2]

A few requested edits.

[leighajarett]

FYI @atsansone and @sfshaza2 -- I made a few updates based on the feedback and a conversation I had with @jmagman yesterday. Feel free to take another look and let me know if you want further changes.

Instead of creating two codelabs, only one will be referenced in this doc. The other, I just put the steps directly in the doc since this isn't a fully supported workflow. Also, I needed to change the images to HTML to get the correct sizing indentation. Feel free to propose an alternative.

@atsansone if you update anything can you leave it as a suggestion so I can make sure that it's all good from a technical perspective? I noticed you added the commit directly to this branch in the last iteration. Thank you!! :)

[atsansone]

@leighajarett : Some comments should be addressed (issue), even if it's just to say Acknowledged. The others are up to you.

Thanks!

[leighajarett]

@atsansone and @jmagman I've gone through all your comments and made adjustments. Feel free to take another look!

[jmagman]

LGTM once @sfshaza2 and @atsansone are happy. Thanks again for doing this!

[atsansone]

LGTM @leighajarett !

[sfshaza2]

@leighajarett, we can't merge this PR until it passes all tests. The linkcheck is failing because the page includes this link: [Adding Home Screen Widgets to your Flutter](http://codelabs.developers.google.com/flutter). The correct link is https://codelabs.developers.google.com/?product=flutter. Can you fix?

[sfshaza2]

Commented out a not-yet-available codelab, fixed a malformed link, indented a note. With these fixes, the CI should build and this can be merged.

[kaciula]

@leighajarett Where can I find the iOS app extension codelab? Or where can I follow its development? I can't currently find it in the available Flutter codelabs.