docs: update LB4 status

[dhmlau]

In the LB4 docs main page, remove the note about LB4 is still in early development and is not yet released.

[bajtos]

Instead of deleting this note, I am proposing to replace it with a note announcing LB 4.0 GA, as we did elsewhere - see e.g. loopbackio/loopback.io#746

{% include note.html content="
LoopBack 4 GA (General Availability) has been released in October 2018,
read more in <a href="http://strongloop.com/strongblog/loopback-4-ga">the announcement post</a>
" %}

[raymondfeng]

CI is failing:

Liquid Exception: Invalid syntax for include tag: content="LoopBack 4 GA (General Availability) has been released in October 2018, read more in <a href="http://strongloop.com/strongblog/loopback-4-ga">the announcement post</a> " Valid syntax: {% include file.ext param='value' param2='value' %} in pages/en/lb4/index.md
jekyll 3.7.3 | Error:  Invalid syntax for include tag:
  content="LoopBack 4 GA (General Availability) has been
released in October 2018, read more in
<a href="http://strongloop.com/strongblog/loopback-4-ga">the announcement
post</a>
"
Valid syntax:
  {% include file.ext param='value' param2='value' %}