add documentation for installing custom root ca certificates

[kosamson]

Adds a documentation page under the "Advanced" section outlining how to install custom root certificates into a container built using ko.

Discussed initially in #1424 and suggested by @jeffmendoza

[kosamson]

question: I thought of something I might have misunderstood while writing this PR.

Can ko users override the value for KO_DATA_PATH and subsequently where ko will bundle the static assets in the built image? I tried doing this myself and could not override the default path of /var/run/ko on a Linux base image.

After looking at the code I think I understand that this value is statically set, and is abstracted via an environment variable just for the sake of running ko on different operating systems.

Is this correct? If so, I will probably change this example to say

equal to /var/run/ko (the default value for $KO_DATA_PATH on Linux platform images)

Also, if I am correct about the value being statically set per OS, should we clarify the values per OS on the static assets documentation page?

[cpanato]

also can use: https:/chainguard-dev/incert after you build your image with ko

[kosamson]

also can use: https:/chainguard-dev/incert after you build your image with ko

This is really useful, I've never heard about this before. Let me add a section on using this in the new documentation page. This makes building a CI workflow to add the certificates automatically seem really easy.

[kosamson]

added a section on using incert (thank you for the suggestion @cpanato).