Use bootstrapped library to generate docs

[nicolasstucki]

We should use the bootstrapped library to generate documentation.

The following line should use dotty-bootstrapped/genDocs
https://github.com/lampepfl/dotty/blob/master/project/scripts/genDocs#L22

But this currently fails with

-- Error: library/src-bootstrapped/dotty/internal/StringContextMacro.scala:582:25 
582 |        checkTypeWithArgs(argument.get, conversionChar, partIndex, flags)
    |                         ^
    |Recursion limit exceeded.
    |Maybe there is an illegal cyclic reference?
    |If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
    |A recurring operation is (inner to outer):
    |
    |  subtype LazyRef(...) <:< LazyRef(...)
-- [E046] Cyclic Error: library/src-bootstrapped/dotty/internal/StringContextMacro.scala:584:28 
584 |        checkTypeWithoutArgs(conversionChar, partIndex, flags, formattingStart)
    |                            ^
    |                            Cyclic reference