Add `pipx` installation to the docs

[PatrickAlphaC]

Describe the desired feature

Slither has very specific dependencies in it's current form, and I've found much greater success working with it in it's own isolated environment. One way to do it is to install slither with pipx instead of pip

Please consider adding this to the docs.

Step 1. Install pipx

Step 2. Install slither

pipx install slither-analyzer

Step 3. Run slither

slither .