Refine README.md

[koppor]

Up to now, https://gi-ev.github.io/LNI was 404 and contained only PDFs, reachable via deep links.

This PR changes that and makes https://gi-ev.github.io/LNI a minimal homepage with links.

• lni-instructions.pdf is not versioned in the repository any more. Just generated and pushed to gh-pages branch
• README.md of the main repository is shortened.

I am not sure, what to distribute to CTAN. Should we distribute docs/README.md also to CTAN? As README-extended.md? :p

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• Fix condition for publishing (publishing is now always done for lualatex, but should be reset to do be done on main only)

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Old

New

The button "Improve this page" directly redirects to an online editor for index.md.

Build workflow:

[koppor]

The refinements of "README.md" should be integrated there. CTAN should be the page to source informations.

It is OK to have the GitHub page hosting the PDFs only. This reduces maintenance effort.