separate kindle and pdf targets

- make `docs` and `html` default
Added Third Party Libraries link to the Community sidebar header on the main page and in the topmost section of the sidebar on the Phobos reference page.
Merge pull request #580 from markisaa/master

Issue 12783: Added 'Third Party Libraries' links to the sidebars
Fix C-style variadics example.
Merge pull request #581 from AndrejMitrovic/FixupVariadics

Fix C-style variadics example.
fix Issue 11396 - Function alias declaration not valid according to spec
fix Issue 12802 - Allow optional 'StorageClasses' for new alias syntax
document implicit conversion rules
Merge pull request #583 from WalterBright/imp-conv

document implicit conversion rules
Merge pull request #582 from 9rnsr/fix_aliasdecl

Issue 11396 & 12802 - Allow optional 'StorageClasses' for old/new alias syntax
Document -boundscheck=[on|safeonly|off]
Begin 2.066 changelog

Start off with -boundscheck= description.
fix Issue 12740 - DMD accepts invalid version syntax
Merge pull request #585 from 9rnsr/fix12740

Issue 12740 - DMD accepts invalid version syntax
fix implicit conversion table for references
Merge pull request #537 from eco/boundscheck

Document -boundscheck=[on|safeonly|off]
Merge pull request #586 from 9rnsr/fix_conversion_table

fix implicit conversion table for references
Merge pull request #518 from MartinNowak/kindleTarget

separate kindle and latex targets
Update GDC home page
Add GDC links to stable sources, make clearer description of downloads page