[FEATURE] Doc improvements

[uhinze]

Is your feature request related to a problem? Please describe.
I came here from the KVS Getting Started Guide and I'm a first-time user. I'm trying to use this SDK only as a GStreamer plugin as suggested in that documentation.

Describe the solution you'd like
Doc improvements:

• Main README:
  • credential-path actually has a default value .kvs/credential that is not documented, the parameter is not mandatory.
  • "For example credential files, see Sample Static Credential and Sample Rotating Credential" - but these are not provided.
  • iot-certificate is also an authentication option I think (judging from the MacOS page
  • AWS_DEFAULT_REGION behavior not documented (default is us-west-2 which is surprising)
• MacOS page:
  • I could not build the project due to cmake not finding OpenSSL. I needed to issue the following command to fix this: export PKG_CONFIG_PATH="/opt/homebrew/opt/openssl@3/lib/pkgconfig