Support DocC generation in Swift Package Index

[MaxDesiatov]

This allows documentation for our package products to be generated and hosted automatically by Swift Package Index.

See this blog post for more details: https://blog.swiftpackageindex.com/posts/auto-generating-auto-hosting-and-auto-updating-docc-documentation

[github-actions]

Time Change: -147ms (0%)

Total Time: 20,470ms

Test name	Duration	Change
Serialization/Call JavaScript function directly	7ms	+3ms (41%)	🚨
Serialization/Assign JavaScript number directly	4ms	-1ms	🎉
Serialization/Call with JavaScript number directly	3ms	-1ms	🎉
Serialization/Write JavaScript string directly	3ms	-1ms (38%)	🎉
Serialization/Call with JavaScript string directly	5ms	+2ms (36%)	🚨
Serialization/JavaScript function call through Wasm import	25ms	+4ms (16%)	⚠️
Serialization/JavaScript function call through Wasm import with int	24ms	+6ms (25%)	🚨

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call from Swift	621ms	+1ms (0%)
Serialization/Swift Int to JavaScript with assignment	445ms	+8ms (1%)
Serialization/Swift Int to JavaScript with call	2,381ms	+5ms (0%)
Serialization/JavaScript Number to Swift Int	662ms	-5ms (0%)
Serialization/Swift String to JavaScript with assignment	511ms	-1ms
Serialization/Swift String to JavaScript with call	2,502ms	+16ms (0%)
Serialization/JavaScript String to Swift String	5,660ms	-3ms (0%)
Object heap/Increment and decrement RC	7,618ms	-180ms (2%)