Improve website documentation

[Marcono1234]

• Improve links & fix dead links
• Improve formatting
• Improve code snippets (formatting & code)
• Fix typos
• Consistently use Docusaurus Admonitions for notes

I hope the changes are rather uncontroversial, but please let me know if you want anything changed or reverted.

I have also added some comments below explaining the rationale.

[Marcono1234]

Removed this link; Jimfs is only one variant of it, and there are already links to Jimfs and ZeroFs on this page.

[Marcono1234]

Old link was a dead link.

[Marcono1234]

The example actually redirects also stderr and stdin, not only stdout.

[Marcono1234]

Wrapped this because the line was cut off / required a scrollbar to be readable.

[Marcono1234]

CI is failing, for example with:

● ApprovalTests › test docs/execution/build-time-compiler.md

    thrown: "WARNING: Using incubator modules: jdk.incubator.vector
    illegal start of expression
            .withMachineFactory(Add::create)
            ^
    illegal start of expression
            .build();
            ^
...

I had assumed the placement of the . at the end of the line instead of the beginning of the next line for wrapped chained method calls was just unconventional formatting, but apparently it was needed to make jbang to be able to compile the code.

Should I just revert these changes for now then?

Side note: It seems in recent jbang versions (e.g. version 0.141.0, due to jbangdev/jbang#2592?) this can be worked around now by adding:

• somewhere at the beginning of the Markdown file

  <!--
  ```java
  class Main {
  public static void main(String... args) throws Exception {
  ```
  -->
  

• somewhere at the end

  <!--
  ```java
  }}
  ```
  -->
  

jbang concatenates all code blocks, and detects the main method and compiles it as regular Java code instead of as jshell snippet then (?).

[andreaTP]

This is very very much welcome @Marcono1234 thanks a lot for doing a full pass with fresh eyes on the docs!

I'll merge as soon as the CI is green, however you want to fix it (with or without main) no strong opinions there.