docs(README): document VHS_NO_SANDBOX

[Delta456]

Fixes #504

[shaunco]

VHS_NO_SANDBOX is not specific to server mode and should be in a different section. Would also be helpful to clarify why/when you'd want to use this, perhaps something like:

When running vhs on Linux as root or with SUID (common for WSL-based Linux on Windows), Chromium may refuse to load due to weakened sandbox capabilities. This issue can be resolved by setting the environment variable VHS_NO_SANDBOX=true, which allows Chromium to run with a weakened sandbox. This enables vhs to function without requiring a non-root or non-SUID account.

[Delta456]

VHS_NO_SANDBOX is not specific to server mode and should be in a different section. Would also be helpful to clarify why/when you'd want to use this, perhaps something like:

When running vhs on Linux as root or with SUID (common for WSL-based Linux on Windows), Chromium may refuse to load due to weakened sandbox capabilities. This issue can be resolved by setting the environment variable VHS_NO_SANDBOX=true, which allows Chromium to run with a weakened sandbox. This enables vhs to function without requiring a non-root or non-SUID account.

How about a separate section for this? Like running on Linux as root or with SUID