Replace old url with new url, show client urls #592
                
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This change updates the document URL and introduces multiple clients in the README.
The document has switched to using GitHub Pages, moving from https://rubydoc.info/gems/line-bot-api to https://line.github.io/line-bot-sdk-ruby/.
Additionally, since finding the various clients without any explanation can be challenging, an introduction has been added in the README. There's no absolute need to add this to the migration guide, as the correct link to v2 code is already provided in the comments of v1 code.
related to: #588