Plugin config directory varies based on OS and should be clearer with docs

[dtzar]

the os.UserConfigDir() varies and therefore isn't the same based on OS.

i.e. on Ubuntu ~/.config/notation/plugins/azure-kv works fine.
on a mac it is ~/Library/Application Support/notation/plugins

[dtzar]

Should be likely updated here: https://github.com/notaryproject/notation/blob/main/specs/notation-config.md

[FeynmanZhou]

Should be likely updated here: https://github.com/notaryproject/notation/blob/main/specs/notation-config.md

@dtzar @yizha1 @notaryproject/notation-release-managers

To avoid duplicate content maintenance work between the website and the code repo, I will migrate all user guides from this repo to notaryproject.dev including https://github.com/notaryproject/notation/blob/main/specs/notation-config.md.

[FeynmanZhou]

@dtzar @yizha1 See https://github.com/notaryproject/notaryproject.dev/pull/80