Add (better) documentation about missing standard Lua modules

[marcelstoer]

This is a sequel to #2461 and @timg11's work maybe re-used here.

Shall we add some extra chapter(s) targeting Lua developers new to NodeMCU? There's the great FAQ by Terry but some questions this audience might have are somewhat buried in there. Questions like:

• Which standard Lua modules are not available on NodeMCU? Our README says right there in the summary but io happens to be missing.
• How does standard Lua functionality X map to the NodeMCU-world, if at all (could contain code snippets)? This list may grow over time.

[stale]

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