[Setup] Clarify the Symfony binary and show small PHP built-in command

[wouterj]

@MaikB on Reddit left some very usefull insights in how we documented Symfony CLI (ref). An excerpt from the comment:

First there is the docs. [...] They just push very hard towards the symfony-cli. That on its own is no big deal. A big deal is that I had to do some digging to even find out that this CLI is closed source. You have to mention that upfront. [... shows the steps it required to find out the source is closed]

Just give people an page which discusses the usage of the built-in web server (php -S localhost:8000 -t public), with or without a router script. Explain the shortcomings which lead you to develop the now deprecated symfony/web-server-bundle. Then tell the story of how the CLI came about and how it is connected to your cloud business. Tell us that you have to keep the sources closed for now, maybe because it contains details which are important for your business to be keep out of the public. Just give us something.

I sincerely do believe this is an communication issue. [...]

Now that I explained myself in more detail, let me say that I am thankful to Fabien and and everyone who has and still is working on Symfony. This is just a problem, not the end of the world. I hope you have a good day, given the pandemic, and stay safe.

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I think this is very usefull information. I personally don't think there is anything wrong with the binary being closed source, but obviously there are people that feel uneasy about it. As we're an inclusive community, I think it makes sense to tell people upfront "hey, this is a closed source tool".
This effectively changes nothing, but would solve part of the problem of these people that now feel excluded. It also opens the possibility to point people to a location to report bugs in the binary (previously, we only mentioned it in the Local web server guide).

I was against documenting the PHP built-in server in the docs, to not make it more complicated. But as it's used by community members and just a very small one-liner, I think it's nice to have a small note about it. Again, it's a small note, but it potentially solves a huge thing for others.

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@fabpot @nicolas-grekas @javiereguiluz @weaverryan As I'm totally not into all strategy and ideas behind Symfony, I would like your opinions/reviews here. The goal at the very minimal is to not upset people with the changes in this PR.

[javiereguiluz]

Thanks Wouter.