Add xk6 documentation

[imiric]

As discussed internally, we should add a high-level page in the docs that explains xk6, what it is, how to use it, etc., without going too much into details about developing an extension or showing any Go code for that matter.

This should include a large visible warning about the experimental stage of the project, no API stability guarantees, etc., and maybe a list of trusted extensions (the same ones mentioned in the upcoming article?) and link to https://github.com/topics/xk6.

[na--]

Just a reminder that k6 v0.31.0 will bring support for output extensions as well, so those should also be documented.

[simskij]

From #338

A more factual/docsy version of what @imiric wrote for the blog at https://k6.io/blog/extending-k6-with-xk6/. Should include:

• How to build an xk6 binary
• How to utilize built-in modules in your script
• Considerations and drawbacks for different running modes, like:
  • Locally
  • Operator
  • Cloud