Merge "Trying Servo" section into official download page

[veyndan]

I recently wanted to give Servo on Linux and macOS a try.

Here was my user flow to download it on Linux:

1. I went to https://book.servo.org/trying/getting-servoshell.html as I had the Servo book already open in a tab.
2. I downloaded the binary for Linux from the page, without reading the intro paragraph (I know that was bad! Probably a common experience though?)
3. I half remembered the command to unzip the binary, resulting in an invalid file.
4. I searched around for what the actual command is.
5. I properly read https://book.servo.org/trying/getting-servoshell.html and realised there was a link to https://servo.org/download.
6. I skimmed that page, searching for the term "linux", and found https://servo.org/download/#linux-instructions.
7. I scrapped what I was doing earlier and just followed those steps.

Here was my user flow to download it on macOS:

1. After my experience from above, I went straight to https://servo.org/download.
2. I downloaded it, and had an issue opening the DMG as macOS was blocking.
3. Luckily for me, there was a step-by-step account of how to bypass this in the same guide.

Here was my user flow to run the resulting binaries:

1. I looked around on https://servo.org/download as my experience thus far was better there than the Servo book.
2. I learnt about --enable-experimental-web-platform-features, but that was it.
3. I had a look at https://book.servo.org/trying/running-servoshell.html and found that much more useful, with keyboard shortcut, auto-launching URLs, and that --help exists in case there's anything that hasn't hit the docs yet.

The result is is that if one doesn't look at both https://servo.org/download and https://book.servo.org/trying/running-servoshell.html, questions will probably arise that were answered by the other source.

In my opinion, we should merge the useful info from https://book.servo.org/trying/getting-servoshell.html and https://book.servo.org/trying/running-servoshell.html, and just have an external link to https://servo.org/download in the sidebar of the book. That way there's a SSOT, and potential setup issues (as evidenced anecdotally above) will be mitigated as all relevant information is in a single place.