Simplify access to library documentation

For users, access to the documentation of a library is not simple enough. See [this comment](https://www.reddit.com/r/cpp/comments/1bdz971/comment/kuq916h/) from Reddit:

> I need to go
> latest release
> documentation
> blob of text with me having to find an inline link in section 4, libraries.
> unordered
> automatic redirect failed
>
> or
>
> learn
> intro? no
> getting started? no
> explore the content? no
> literally any of these links is useless. completely.
>
> find libraries on top
> unordered, but in a hard to read list
> documentation
> automatic redirect failed

A suggestion to fix this issue would be to reserve the top section of the main page to the most popular use cases, like downloads and documentation, and put "why use boost", "mission" and other information that will be read just by new users in a less visible part (.e.g. lower in the page or in a different page).

The documentation link can be put nearly at the same level as the current "Download the latest release" that says "Documentation for the latest release" that would redirect to something like "https://www.boost.io/doc/libs/boost_1_84_0/libs/libraries.htm". Alternativelly, we can have a drop-down or similar menu in the front page instead of the "Documentation for the latest release" button where a user selects a library (e.g. Unordered) and the selection redirects to the [documentation of the latest release of Unordered](https://www.boost.io/doc/libs/boost_1_84_0/libs/unordered/doc/html/unordered.html).