Add "external" icon to links to other domain

[adriendupuis]

Question	Answer
JIRA Ticket	N/A
Versions	All
Edition	All

While the CSS is there, the JS to add the icon wasn't. Piece of code taken from https://github.com/ibexa/documentation-developer/blob/c636be472cab8f702d82f6ce3b0b2273beb6e916/docs/js/custom.js#L109-L113

Previews:

• SeenThis! external link
  • https://doc.ibexa.co/projects/userguide/en/latest/content_management/block_reference/#seenthis-block without icon
  • https://ez-systems-developer-documentation--324.com.readthedocs.build/projects/userguide/en/324/content_management/block_reference/#seenthis-block with an icon
• Max tokens definition's "word" external link
  • https://doc.ibexa.co/projects/userguide/en/master/ai_actions/work_with_ai_actions/#edit-existing-ai-actions
  • https://ez-systems-developer-documentation--324.com.readthedocs.build/projects/userguide/en/324/ai_actions/work_with_ai_actions/#edit-existing-ai-actions with an icon (which pushes the quotation mark, maybe the quotation marks could be removed or moved inside the link).

Checklist

• Text renders correctly
• Text has been checked with vale
• Description metadata is up to date
• Redirects cover removed/moved pages
• Added link to this PR in relevant JIRA ticket or code PR

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mnocon]

Thanks!