Docs: access token and new endpoints (/login, /userinfo, /v2/logout)

[shawnhankim]

Background:

• Access token is added in the NGINX Plus OIDC.
• New endpoints are added in the issue of #55
  • /login endpoint
  • /userinfo endpoint
  • `/v2/logout endpoint
• Quick start guide is linked into this link to minimize the change of files. It is subject to change by creating a branch (e.g. demo) in the repo of https://github.com/nginxinc/nginx-openid-connect.
• The docs and comments of configuration for the above new features need to be enhanced for customer convenience.

AC:

• Main Docs: Enhance https://github.com/nginxinc/nginx-openid-connect#readme
• Demo Docs: Enhance https://github.com/nginx-openid-connect/nginx-oidc-examples/tree/main/001-oidc-local-test
• Comments: Enhance comments of configuration files:

  - frontend.conf
  - openid_connect_configuration.conf
  - openid_connect.server_conf
  - etc.