Prepare the environment of Sphinx documentation (Issue 138)

http://trac.xapian.org/ticket/138

- Preparing the environment
- Integrating "getting started" documentation within it
- Automating the version 
- Link it to the global building system
- Apply the same on python 3

You can browse how it looks here: http://xapian.alfanous.org/

what's next? 
- Check building doc errors
- An extension for sphinx to include function declared by assignment

```python
MultiValueKeyMaker.add_value = new_instancemethod(_xapian.MultiValueKeyMaker_add_value,None,MultiValueKeyMaker)
```

Dependencies:
python-sphinx 
We really need a variable (e.g. `RST_DOCS`) which lists the actual rst sources, then use that here instead of the wildcard, and include `$(RST_DOCS)` in `EXTRA_DIST` above instead of just removing `index.rst`.  Without the latter, the rst sources won't be included in the tarball of sources.