You don't have to read it all but as reference if you're not sure how an API works or other things like that, you will probably find the details in the official doc.
For rotten links, well ... it happen everywhere on the internet
try your luck with web archives wayback machine:
You can still do it with Animate CC but imho it is not the best way to do once your program become quite big (it's like the goold old days of AS2, AS3 can do much better).
Usually what I advise is to build the assets in Animate CC, export them to SWC with named symbols
and then use the external SWC as an external library in Flash Builder (or any other IDE) to do the programming.
But really it depends on the project itself.
So Flex is 3 things really
- the IDE, Flex Builder, which got renamed to Flash Builder
not update since last version 4.7 but you can work with it for most projects
- the SDK, which in the mean time got donated to the Apache Foundation
and is still regularly updated see flex.apache.org and github.com/apache/flex-sdk
- the Flex tooling (mainly command-line tools) like
- the Flex libraries/frameworks
eg. when you build your app based on MXML and Flex components
So yeah Flex is still used and is also evolving (see for ex: Apache Royale Coming Soon).
Now the thing to understand if you can have 2 paths depending on which SDK you choose
- either use the Flex SDK (either old Adobe Flex SDK or new Apache Flex SDK)
that you can merge with the "old" AIR SDK (based on ASC1)
- or use only the "new" AIR SDK (based on ASC2)
almost the same but it uses a new compiler
The differences in compilers are not that important (unless you badly need some specific things)
and technically you can use both the compilers, for ex: you could use a SWC compiled with ASC2
in a project that use the Flex SDK merged with AIR SDK (ASC1).
If you use something like Flash Builder and need to use different versions of the Flex/AIR SDK
it is easier to use the "Flex SDK merged with AIR SDK", but really a detail if you build only 1 project for 1 platform.
That said, I would say knowing how to setup both is a plus for a dev.
For the technical, here the differences using ASC 2.0
In short ASC2 is a stricter compiler and allow for inlining and dead-code elimination,
but it can confuse a lot of dev when it comes to compile font and/or writing strict AS3.
Yeah well SWF on the web is soon to be dead since Flash EOL has been announced for 2020,
you can still work with SWF files and use HTML that embed SWF on the desktop etc.
see for ex AIR, HTML and SWF, but SWF running inside the default browser on the user's desktop is a dead end.
If your ultimate goal is to produce games for the web, Flash/SWF is not anymore the solution.
Maybe Apache Royale will bring a solution to that in the coming years,
maybe you can ditch the web and produce only for desktop and/or mobile,
or maybe you can ditch AS3/Flash/AIR and move to HTML5/CSS/JS (which imho is a dead end too for other reasons).
In fact, tech move really fast and it's pretty hard to predict what will work nicely in the futur,
only thing I can say is if you plan to produce a small'ish Flash game to publish as SWF on the web,
it can still work now, but expect it to become harder and harder for your users to play it from now to 2020, and in 2020 it will be technically impossible as all major browsers will have removed altogether the flash plugin.
But the same you can take an "old" SWF game sources and update it to publish on mobile with AIR,
you can also plan your new SWF project the same way.