Update plugin docs for new ORM and other changes.

Update the docs to reflect changes in the ORM. Clean up formatting and
a few other small issues. requestAction doesn't have the same
limitations as it used to have.
Add example of how TableRegistry should work with plugin tables.
Fix up docs based on review feedback.
Fix dash.
[fr] update docs for 3.0

folder /cakephp-overview
[fr] Follow 810e398eeea0d540196e42721290f11d6d7e2a8e

Remove docs for code that no longer exists.
[fr] Follow 2742af942177ffbdb8f6576e863efc297c830f20

Remove examples and tutorials from the TOC
[fr] Follow 06abe88671825e3e6b0e2ebfd6d61b543ffa98e5

Remove references to scaffolding.
[fr] rearrange tocs to follow en
[fr] Follow 91702c053bfae5ff14ce47e3c6d69ff7d6f3c9bd

Fix more broken links.
[fr] Follow en 3.0 docs

root folder
Merge pull request #1024 from cakephp/3.0-plugin

3.0 Update plugin docs
Fixes to plugin docs
Fix Composer, PEAR, and GitHub casing