test

Add docs for new `@mustBeConst` diagnostic #704

Job	Run time
Build site and check links	1m 2s
Check if code excerpts are up to date	42s
Validate Firebase configuration	28s
Analyze and test code examples (beta, false)	3m 7s
Analyze and test code examples (stable, false)	3m 37s
	8m 56s

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