Prepare gem for RubyGems release with CHANGELOG and release docs - v0.2.0

[julioalucero]

Jira ticket

FR-740: Fix nofollow on internal links
(There is no ticket, using FR-740 branches, but I'm open to do it in a a new one)

PR description

Sets up the gem for publishing to RubyGems, ensuring proper version tracking.

• Add CHANGELOG.md following the next_rails format, with v0.2.0 and v0.1.0 history.
• Change the README install line from github: to a plain RubyGems gem '...'.
• Add a "Steps to release a new version" section to the README.
• Point changelog_uri at the actual CHANGELOG.md.
• Fix spec.files: shipped MIT-LICENSE (doesn't exist, instead file is LICENSE.txt), so the gem was packaged with no license, also add CHANGELOG.md
• Replace deprecated add_runtime_dependency with add_dependency.

Test suite

This gem has no test suite. I opened PR #6 with one, but heads up: I used AI to write it (then reviewed it) to save time, since building it by hand wasn't scoped or approved for this work.

I'm totally fine writing it manually with less AI, it just needs the time/effort approval. So treat PR #6 as optional: if we don't like it, or it'd take too long to get right, we can just close it and do it properly later.

QA steps

1. gem build jekyll-external-link-accessibility.gemspec => should build with no warnings.
2. After merge:

• tag 'v0.2.0',
• push tags, then
• gem push

[julioalucero]

add_runtime_dependency is obsolete