[Don't merge yet] Replace old getting started flow with two new flows

[parlough]

Staged: https://flutter-docs-prod--pr12341-feat-streamlined-get-starte-hukqrcpn.web.app/get-started

Replaces the old getting started flow (currently at https://docs.flutter.dev/get-started/install), with a new getting started flow that's built around a quick start (happy path) with a fallback that references other docs.

To enable this, we've created new standalone installation docs and "set up development" docs, currently linked to from /install#get-started.

"Get started" and "Set up" buttons and links will lead to /get-started which has two options to choose from:

1. Quick start - /get-started/quick

   This option is highlighted and recommended. It provides a streamlined approach for installing and setting up Flutter by using VS Code (or other VSX-supporting Code OSS-based editors) and only setting up web development. The steps it follows are similar to this:

   1. Changes content based on the developer's current OS, with fallback buttons to switch.
   2. Install VS Code and Flutter plugin
   3. Use Flutter plugin to install and set up Flutter SDK
   4. Use VS Code plugin to create new Flutter app, run it on the web, try hot reload on the web, and try out the Flutter sidebar.
   5. Directs developers to the "Write your first Flutter app" codelab and FWE.

2. Custom setup - /get-started/custom

   Doesn't cover installing Flutter on the page, but rather relies on the other documentation we've added.

   1. Gives the developer a choice to install with VS Code or manually.
   2. Suggests developers set up an IDE or editor with Dart and Flutter support if they'd like.
   3. Provides options for target platforms that can be set up.
   4. Directs developers to the "Write your first Flutter app" codelab and FWE.

Note we do expect this to be changed and streamlined further once the interactive FWE tutorial is further along.

Staged: https://flutter-docs-prod--pr12341-feat-streamlined-get-starte-hukqrcpn.web.app/get-started

---

Resolves #11911
Fixes #11672
Fixes #12318
Closes #12318
Closes #10957
Closes #11109
Closes #11405
Closes #11807
Closes #12309

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 68ad5e8):

https://flutter-docs-prod--pr12341-feat-streamlined-get-starte-hukqrcpn.web.app

[mit-mit]

Hi Parker, I'm about to go on vacation for a week. Can this wait until after that?

[parlough]

Of course. Enjoy your vacation @mit-mit! No rush on this :)

[sfshaza2]

I know this is still in draft, but lgtm

[sethladd]

LGTM , thank you

[ericwindmill]

LGTM (Parker and I talked about this in our previous 1:1.)