Nox session `release:prepare` requires environment variable GITHUB_TOKEN to be set

[ckunki]

PTB User Guide already describes the nox session release:prepare, however some details should be added

• In case CLI option --no-pr is not specified, PTB tries to create a PR on GitHub.
• For accessing GitHub, the PTB needs a related GitHub Token, passed via environment variable GITHUB_TOKEN
• If the user did not set this variable, they get an error like

  pull request create failed: GraphQL: 
  Resource not accessible by personal access token (createPullRequest)
  

Options

• O1) Remove the option to create the PR on the fly and instruct users to create the PR manually on GitHub
• O2) improve documentation to explain this and add a more precise error message,
  e.g. please set environment variable GITHUB_TOKEN