RubyReference changelog

2020-01-07 -- Ruby 2.7 update

The whole site is re-rendered from ruby_2_7 branch of Ruby repo. Notable changes:
- Pattern matching docs are added (in the absence of official docs, just a copy-paste of my own PR to Ruby itself)
- Removed docs for stdlib parts that were dropped: scanf, shell, profiler
Minor logical restructuring (bundler moved under Developing/Libraries, Fiber under Concurrency, etc.)
Rendering and validation scripts are finally rewritten into proper Rakefile from a bunch of ad-hoc *.rb executables;
A lot of small rendering bugs are fixed in the process.

The whole site is re-rendered from ruby_2_6 branch of Ruby repo. Notable additions that made it into the reference rendering:
- RubyVM::AbstractSyntaxTree;
- Enumerator::ArithmeticSequence, Enumerator::Chain;
- Endless ranges, timezones for Time
Other additions:
- "Ruby in 20 minutes" from the site
- RubyVM (was not rendered for 2.5 because was not documented in Ruby repo)
- Enumerator::Lazy (still has no documentation in Ruby repo, custom content written)
- File::Stat and File::Constants (were missing in prev. version of reference due to bug)
- Bundler in a standard library (custom content)
- New links in Appendix B
Lot of small content fixes and cleanups, including Since Ruby (version) badges besides some of functionality (incomplete, mostly catching what's new since 2.6, for reference to stay relevant and helpful for those who uses earlier versions)
A bit of visual cleanup, including copyable links to sub-headers.
Patreon link :)