Add nanoc 4 upgrade guide (WIP)
List removed features
Add more content to upgrade guide
Describe identifiers and globs, and move sections around
Merge branch 'master' into feature/nanoc-4-upgrade-guide
Add link to upgrade guide
Clarify why update comand was removed
Add nanoc-core 4.0 API documentation
Add link to Nanoc::Identifier API docs in upgrade doc
Do not include `update` command in to-be-revisited list
Remove view command from list of removed features

This decision has been reverted, as nanoc view is quite useful.
Expand compact rules section in upgrade guide
Run admonition filter on HTML, not Markdown
Simplify #generate
Mention index filenames no longer being stripped
Highlight pattern change
Filter out smartness in KBD
Clarify why Ruby 1.8.x support was dropped
Expand example Rules file
Remove obsolete TODO