Yndira/fix eleventy excerpts

[Yndira-E]

Description

This builds on top of the Dependabot Eleventy upgrade and fixes collection templates that were still reading templateContent too early.

Eleventy 3 is stricter here and throws TemplateContentPrematureUseError for some collection items, especially the ones that rely on content before <!--more-->.

It also fixes a pre-existing broken internal link caught by the link checker.

Changes

• configured Eleventy front matter excerpts to use <!--more-->
• stopped using templateContent for collection excerpts in list templates
• added a shared summary.njk partial for summary rendering
• removed the old custom excerpt filter
• made truncate tolerate empty values
• fixed a broken internal blog link in dashboard-0-1-release.md

Why

This preserves the current editorial workflow where <!--more--> defines the excerpt, while making it compatible with Eleventy 3.

Related Issue(s)

Checklist

• I have read the contribution guidelines
• I have considered the performance impact of these changes
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
• For blog PRs, an Art Request has been created (instructions)