Add hints on custom domains with Github Pages and Next JS

[tordans]

Thanks for this workflow. I just used it with a custom github domain and those are the changes I made:

• Add the /CNAME file
• Omit the basePath part
• Omit https://github.com/gregrickaby/nextjs-github-pages?tab=readme-ov-file#add-base-path-to-pagetsx

See https://github.com/osmberlin/osm-traffic-sign-tool/blob/main/.github/workflows/deploy-github-pages.yml

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Another think that would fit well into the readme is, that it looks like this workflow is suggested by Github when one chooses Settings > Pages > Build and deployment / Source > Github Actions. Only after a short while this UI picks up on the Action I pushed manually.