Improve docs

[ShapelessCat]

Since in some files I inserted some empty lines to separate code examples and its surrounding context paragraphs,
to check what I changed besides white spaces, please turn on "Hide whitespace changes".

• Current view of an ordered list in the match-syntax.md is wrong, and I adjust the indentation to fix it.
• Since NotGiven is new in Scala 3, I think in its example code use the new summon is better than implicitly, and I did this replacement.
• Add missing with in the declaration of given instances.
• Use inline instead of final to declare constant expressions.

[michelou]

@ShapelessCat The insertion of empty lines to separate code examples and their surrounding context paragraphs is a good thing IMO. I made a few insertions too, although not systematically as you did. Docs contributors should adopt that rule for future changes.

[michelou]

@liufengyun Ok for me.

[liufengyun]

Thanks for your contribution @ShapelessCat 🎉