Add new Introduction and Advanced Usage sections.
Spell out a few more details, like "ninja install".
Improve the layout around example commands.
Fix grammatical errors and tighten up the text.
Explain the --prefix option.

v2: Remove language about 'ninja clean' and move link to Meson
information about separate build directories earlier in the page.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>