DOC-3384 - Fix broken NPM and Yarn links on docs homepage

[kemister85]

Ticket: DOC-3384

Site: Links to affected pages:

• TinyMCE 8 Documentation – Integration Options (NPM and Yarn links)
• Introduction to TinyMCE – Integration Options (NPM and Yarn links)
• Self-hosted installation – Package managers table
• Create a Skin – Prerequisites

Changes:

• Fixed NPM and Yarn link syntax in modules/ROOT/partials/what-is-tinymce.adoc: corrected AsciiDoc caret placement for new-tab behavior ([text^] not ^[text]) and updated Yarn URL from classic.yarnpkg.com to yarnpkg.com/package/tinymce.
• Fixed link syntax in modules/ROOT/pages/installation-self-hosted.adoc: npm, Yarn, Composer (PHP), and NuGet (.NET) in package manager table.
• Fixed link syntax in modules/ROOT/pages/creating-a-skin.adoc: Node.js, Yarn, Less, and section-on-variables in Prerequisites.

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Pre-checks:

• Branch is correctly prefixed: hotfix/8/DOC-3384
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files have been included where required (if applicable).
• Files removed have been deleted, not just excluded from the build (if applicable).
• Files added for New product features include a release note entry.
• Major or minor version changes have updated the supported-versions.adoc table.
• Build passes without console errors, warnings, or issues.