Docs for 0.12.0

[lihaoyi]

This is a start for documenting most of the things we need to document in 0.12.0, with some refactoring of the docsite in general. More to do, but this should be a start. In particular I want to write up a section on writing plugins and testing them using mill-testkit, and sections on adding new language toolchains to Mill

• Bug fixes for java/scala example doc inheritance ignoring leading text

• Break out Testing Java Projects and Testing Scala Projects out of *Build Examples and *Module Config. I intend to break out Publishing * Projects and Linting * Projects as well, but that will take more work to flesh those out

• Broke up Extending_Mill.adoc, with various parts of it end up in Modules.adoc, Tasks.adoc, and a new Import_File_And_Import_Ivy.adoc and The_Mill_Meta_Build.adoc

• Consolidated the section on Mill Plugins with Extending Mill. I hope to add a section on Writing/Publishing Mill Plugins as well, but that can come in a follow up

• Replaced the old Foreign Modules documentation with a new example doc for multi-build-file (now renamed to 10-module-sc)

• Added a bunch of /** See Also */ snippets to the 5-resources example so people can see the relevant code without needing to click around the folder on github

• Added a section on code change invalidation to the 2-primary-tasks.

• Added mention of sandbox folders to 2-primary-tasks and 7-modules examples

• Updated the changelog in readme.adoc

• Other small misc tweaks