docs(README): add note and links to update ruby version

[XAJX179]

Because

ruby version can cause error.

This PR

• adds a note to README and first spec.
• adds link to updating ruby content.
• adds link to setup local ruby version.

Issue

Related TheOdinProject/curriculum#30470 (comment)

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. String spec: Update instructions for clarity
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes changes in the spec folder, they are also updated in the corresponding file in the spec_answers folder (with passing tests).