Documentation: Add a link to the keywords list

[pravic]

As per this request:

Add the following link (/std/index.html#keywords) next to the existing ones:

See all std's items
Re-exports
Primitive Types
Modules
Macros
Keywords <-- this one.

cc @GuillaumeGomez

[GuillaumeGomez]

Thanks for opening the issue! I'll send the PR tomorrow (but if you want to do it, please go ahead 😃).