Allow people easier access the source code, raise issues and read the changelog, by adding [project metadata](https://guides.rubygems.org/specification-reference/#metadata) to the gemspec file.

These links will appear on the RubyGems page at https://rubygems.org/gems/annotate and be available via the RubyGems API – after the next release.

Also: used https in URLs.

In #764, it was pointed out that the gemspec requires ruby version `>= 2.2.0`. We've stopped testing for older versions of Ruby in CI and so it makes sense to make it explicit.

* Convert documentation files to Markdown
* Add notice about models not being annotated by default to changelog in 3.0

Updates the requirements on [rake](https://github.com/ruby/rake) to permit the latest version.
- [Release notes](https://github.com/ruby/rake/releases)
- [Changelog](https://github.com/ruby/rake/blob/master/History.rdoc)
- [Commits](https://github.com/ruby/rake/compare/v10.4.0...v13.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Before, whenever a new file is added to `lib`, the contributor or
maintainer has to remember to explicitly add it to the gemspec. If this
step is forgotten, it results in a broken release, like #657.

This commit makes this process less error prone by using `git ls-files`
to automatically include all appropriate files in the gem. This is in
line with what is advocated by Bundler's gem template:

https://github.com/bundler/bundler/blob/2a4d88a7044528209321fd4441e995ec66f604b9/lib/bundler/templates/newgem/newgem.gemspec.tt#L26

Currently the gem released as version 3.0.0 is broken. Fixes #657 

The gem is missing constants.rb and parser.rb file

The RubyGems gemspec property `rubyforge_project` has been removed without a replacement.

Refer to https://github.com/rubygems/rubygems/pull/2436

* Drop support of Ruby < 2.2

Fix bug introduced by #469.

Fixed some offenses