Mention any_of as alternative to enum

[yannham]

Prompted by #1993 (comment).

Since enum variants can't be (de)serialized, they aren't good candidates for values that can go in a configuration. This commit makes the documentation on this point clearer, and points to the alternative untagged union std.contract.any_of.

I can't request reviews for people outside of the Tweag org, but cc @rben01 if you'd like to have a look.

[github-actions]

Bencher Report

Branch	doc/enum-variants-mention-anyof
Testbed	ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	nanoseconds (ns)
fibonacci 10	📈 view plot 🚷 view threshold	488,220.00
foldl arrays 50	📈 view plot 🚷 view threshold	1,648,000.00
foldl arrays 500	📈 view plot 🚷 view threshold	6,722,500.00
foldr strings 50	📈 view plot 🚷 view threshold	7,013,000.00
foldr strings 500	📈 view plot 🚷 view threshold	61,545,000.00
generate normal 250	📈 view plot 🚷 view threshold	41,763,000.00
generate normal 50	📈 view plot 🚷 view threshold	2,055,100.00
generate normal unchecked 1000	📈 view plot 🚷 view threshold	3,277,300.00
generate normal unchecked 200	📈 view plot 🚷 view threshold	763,550.00
pidigits 100	📈 view plot 🚷 view threshold	3,158,900.00
pipe normal 20	📈 view plot 🚷 view threshold	1,499,000.00
pipe normal 200	📈 view plot 🚷 view threshold	10,036,000.00
product 30	📈 view plot 🚷 view threshold	819,790.00
scalar 10	📈 view plot 🚷 view threshold	1,491,000.00
sum 30	📈 view plot 🚷 view threshold	806,450.00

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