Add external link icons and target="_blank" to all external links in documentation

[Copilot]

This PR updates all external links in the PMM documentation to improve user experience by:

1. Adding visual external link indicators using the :octicons-link-external-16: icon
2. Making external links open in new tabs with the {:target="_blank"} attribute

Problem

External links in the documentation were inconsistent - some opened in the same tab (taking users away from the docs), and there was no visual indication that links were external. This could confuse users and disrupt their documentation browsing experience.

Solution

Updated all external links to follow the standardized pattern:

[Link Text :octicons-link-external-16:](https://example.com){:target="_blank"}

Changes Made

• 134 external links updated across 34 documentation files
• All external HTTPS links now display with the external link icon
• All external links open in new browser tabs
• Fixed Unicode quote encoding issues in some existing target attributes
• Verified documentation builds successfully with MkDocs strict mode

Examples

Before:

[Percona Community Forum](https://forums.percona.com/c/percona-monitoring-and-management-pmm/30/all)
[CVE-2025-26701](https://cve.mitre.org/cgi-bin/cvename.cgi?name=2025-26701)

After:

[Percona Community Forum :octicons-link-external-16:](https://forums.percona.com/c/percona-monitoring-and-management-pmm/30/all){:target="_blank"}
[CVE-2025-26701 :octicons-link-external-16:](https://cve.mitre.org/cgi-bin/cvename.cgi?name=2025-26701){:target="_blank"}

Visual Result

External links now clearly display with a visual indicator (arrow-out icon) and open in new tabs, ensuring users stay on the documentation site while still being able to access external resources.

Testing

• ✅ Documentation builds without errors using mkdocs build --strict
• ✅ All external links display correctly with icons in the generated HTML
• ✅ External links open in new browser tabs as expected
• ✅ No broken links or formatting issues introduced

Original prompt

create a pr that changes all external links to open in a new tab and adds this icon as in the example below: Percona Operator for PostgreSQL :octicons-link-external-16:

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[it-percona-cla]

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[percona-robot]

🚫 [linkspector] _{reported by reviewdog 🐶}
Cannot reach https://developer.hashicorp.com/nomad Status: 429

[percona-robot]

🚫 [linkspector] _{reported by reviewdog 🐶}
Cannot reach https://www.nomadproject.io/ Status: 429

[percona-robot]

🚫 [linkspector] _{reported by reviewdog 🐶}
Cannot reach https://developer.hashicorp.com/nomad/docs Status: 429

[percona-robot]

🚫 [linkspector] _{reported by reviewdog 🐶}
Cannot reach release-notes/{{release}}.md Status: 404 Cannot find: release-notes/{{release}}.md

pmm/documentation/docs/index.md

Line 3 in 5f7c99a

<i info>:material-information: Info:</i> This is the documentation for the latest PMM 3 release. For details, see the [PMM {{release}} release notes](release-notes/{{release}}.md).