adds kramdown to dev dependencies

Yard's default use of the rdoc markdown provider was choking on links
containing codeblocks, for example: [`ENV`](link/to/somewhere). These
style links are pretty popular in the text details provided with semconv
attributes.

Let's switch to kramdown which is a markdown parser and
renderer that:

* can handle inline code blocks inside a link anchor
* is pure Ruby (no C extensions) so works with JRuby

semantic_conventions/.yardopts

@@ -1,6 +1,7 @@
 --no-private
 --title=OpenTelemetry Semantic Conventions
 --markup=markdown
+--markup-provider=kramdown
 --main=README.md
 ./lib/opentelemetry/**/*.rb
 -

semantic_conventions/opentelemetry-semantic_conventions.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'opentelemetry-api', '~> 1.0'
 
   spec.add_development_dependency 'bundler', '>= 1.17'
+  spec.add_development_dependency 'kramdown', '~> 2.3'
   spec.add_development_dependency 'minitest', '~> 5.0'
   spec.add_development_dependency 'rake', '~> 12.0'
   spec.add_development_dependency 'rubocop', '~> 1.3'