Introduction: what do we have in Boost?

**Which documentation does this apply to?**
User's Guide

I think that the current introduction doesn't transmit the user why she should use Boost. For example:
```
Particular strengths of the Boost libraries include smart pointers, unit tests, lambda functions, function binding, regex, a file system, to name a few.
```

We already have all of this in the standard, so this is somehow contradicting the other arguments.

I'd say the table on library categories belongs to a reference section, rather to an introduction section, since it's ordered alphabetically and doesn't provide a way for the user to explore the libraries inside each category.