Add CSS source maps reference to docs. Closes #15211

This rather trivial PR adds relevant information about CSS source map to "What's included" documentation section and it:
- updates distribution file list with relevant files
- adds a short reference to CSS source map purpose.
```
bootstrap/
├── css/
│   ├── bootstrap.css
│   ├── bootstrap.css.map
│   ├── bootstrap.min.css
│   ├── bootstrap-theme.css
│   ├── bootstrap-theme.css.map
│   └── bootstrap-theme.min.css
```
```
[...] and minified CSS and JS (bootstrap.min.*) and CSS source maps (bootstrap.*.map) for use with web browsers developer tools. 
Fonts from Glyphicons are included, as is the optional Bootstrap theme. [...]
```
This PR is not trying to make documentation capable of fully answering questions like this one:
http://stackoverflow.com/questions/21504611/what-are-the-map-files-used-for-in-bootstrap-3-1