Add `--generate-link-to-definition` option when building on docs.rs

[GuillaumeGomez]

This option generates links in source code pages, allowing to jump to definition and to jump back to doc. It makes browsing the source code pages much nicer. You can see it in action here.

[coveralls]

Pull Request Test Coverage Report for Build 5555616446

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 62.423%

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Totals
Change from base Build 5231977364:	0.3%
Covered Lines:	1819
Relevant Lines:	2914

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💛 - Coveralls

[Geal]

thanks!