Guide for Configuring Snippets in NGF

[mpstefan]

As a user of NGF with some knowledge of NGINX configuration
I want a guide to show me how to insert the NGINX configuration I need for my route rules
So that I can take advantage of NGINX features not yet exposed through a first-class Gateway API extension.

Acceptance

• A guide to configuring snippets via SnippetFilters is created which contains:
  • A note about if any nginx directive fails to be applied to create an issue on the NGF repo.
  • A recommendation at the top of the guide that guides users to use our first-class policies before using snippets.
  • Security concerns around enabling snippets and why they need to be enabled.