add instructions to generate GITHUB_TOKEN

[gitcommitshow]

Add instructions in docs about the scope and preferred method of generating this PAT.
GITHUB_TOKEN is needed to read file content of the docs pages and code. It also needs to search (which I don't believe requires any additional permission).
The testing is working fine with a fine-grained pat with no additional permission. It is able to access public code and search.

[contributor-support-github-app]

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