Avoid auto in docs introduction

[akrzemi1]

The initial docs examples use auto as i n:

auto r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);

I recommend spelling out the return type:

boost::charconv::from_chars_result r = boost::charconv::from_chars(buffer, buffer + std::strlen(buffer), v);

It will make it clear for the person that first reads the introduction that the return type is also part of this library, as opposed to using another Boost library or a component from STD.