Shorten and simplify the landing page.

Keep it short and sweet with all the same great links.
Update blog article a bit.
Improved wording, fixed typos
Composer with capital C

https://getcomposer.org/doc/00-intro.md
Composer with capital C for other languages, too
Fixed/improved wording
Merge branch '3.0-proof-tutorials' into 3.0

Conflicts:
	en/plugins.rst