Add link to the Vec<T> API documentation

[Vinnl]

I was looking for this link while working my way through the book. I'm not sure if this is the correct way to link to API documentation (as its not included in this repo), but it looks good to me.

[steveklabnik]

I think this would be fine to add, but I'm not sure we've decided a like, policy for when to link and when not to. Carol, what do you think?

[tannakartikey]

@steveklabnik @carols10cents it would help if there was a policy. I was going through the docs today while I thought that at some places links were needed and found this issue.

For example, I am going through Generating a secret number docs. In the first paragraph itself (at the end) the link of "rand crate" is given.

After a few scrolls, I was reading "Generating a Random Number" where rand is mentioned a few times and methods like rand::thread_rng but there are no links. I had to search and find the docs.

And just a suggestion for the policy. How about giving links to all the methods even though they are repeating provided we can automate the process while building docs.
Let's assume if we were able to achieve that then reading thorough the docs will not be quite easy on eyes. To solve that we can remove the traditional hyperlink styling (blue color and underline) and keep it as it is now (without the hyperlink).

I can contribute in any way according to the policy.

[carols10cents]

I'm fine with this; I think I'm fine anywhere someone wishes there was a link. I don't think we're going to transfer these to written-out URLs in print though.