add documentation about air gap installation support

[developer-guy]

What would you like to be added:

We had talked with @luhring @wagoodman related to this and decided to add documentation about the air gap installation support of Grype.

Why is this needed:

To give information to people who might be interested in setting up Grype in an air gap environment.

Additional context:

[developer-guy]

Found this.

https://github.com/anchore/grype#offline-and-air-gapped-environments

[luhring]

Let's re-open this, with the goal of providing better guidance to folks working in air-gapped environments

[luhring]

I think there might be two separate workflows we could outline for users:

1. Fetching the database in advance, and ensuring that it's placed in Grype's cache directory ahead of running Grype.
2. Letting Grype fetch the database on-the-fly, but using a different endpoint for retrieving the database (such as on a private network). Here, we can talk about customizing the database update URL and configuring Grype to use a custom CA (if needed).

[spiffcs]

@developer-guy is the current documentation sufficient, or do you have any other suggestions for improvement? Otherwise I think we can close this issue

[kzantow]

Since this hasn't had a response in a while, and there seems to be sufficient documentation, I'm going to close this issue. But also note that we have plans to publish a docker image with embedded database as well, which may help air-gapped installs too.