Adds 'with' to the bootstrapping docs to help clarify how to build a new compiler

[martinomburajr]

The sentence

However it was abandoned long ago and the only way to build a modern version of rustc is a slightly less modern version.

feels like it's grammatically missing the preposition 'with', as well as some commas around "However" etc.

[tshepang]

this is a good change, and it exposes text that is almost exactly duplicated... are you happy to remove/reduce that, or would you rather that happen in a different pr

[tshepang]

will defer that to a future pr... no need to delay this

[martinomburajr]

@tshepang thanks for the review 🙏 . I had noticed the paragraph is duplicated word for word across both files. I had thought about removing the second occurrence, but wasn't sure if this was intended.

If we were to go forward with this, the two options that make sense to me are:

1. Summarize what was in src/building/bootstrapping/intro.md into src/building/bootstrapping/what-bootstrapping-does.md
2. Completely remove the duplicate paragraph in what-bootstrapping-does.md

Let me know what makes sense and I can make the quick change.

[tshepang]

or 2a, add a link to that bit of text
I am curious what a summary would look like though... the text is short already
feel free to decide