feat: enforce alt attributes and add image accessibility docs

[htnabe]

Overview

This PR enforces explicit alt attribute output for rendered images and adds dedicated image accessibility guidance documents.

Changes

• Added warning-based alt checks in markdown image rendering.
• Updated image templates/shortcodes to always output alt attributes.
• Added dedicated guidance docs for image accessibility in English and Japanese.
• Kept getting-started guides as quick-start documents and linked to the detailed accessibility docs.
• Updated README documentation index with new guidance links.

Related Issues

• N/A

Checklist

• Self-review done (following review.prompt.md)
• Hugo production build passes: cd exampleSite && hugo --gc --minify
• Docs updated if behavior changed
• No unrelated changes included
• No unnecessary commented-out code included

Notes for Reviewers

Current enforcement is warning-based to avoid immediate breaking changes. Future tightening to build errors can be done after migration guidance.

[Copilot]

Pull request overview

This PR improves image accessibility across the HikaeMe Hugo theme by ensuring rendered <img> elements always include an alt attribute, introducing warning-based enforcement for missing/empty alt text in some render paths, and adding dedicated documentation for image accessibility (EN/JA) with links from the getting-started guides.

Changes:

• Updated multiple templates/shortcodes to always output alt attributes for images.
• Added warning-based checks during Markdown image rendering and shortcode usage to surface missing alt text.
• Added new image accessibility guidance docs (English/Japanese) and linked them from existing getting-started guides.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
layouts/shortcodes/img.html	Adds alt/caption fallback logic + warning for missing alt; now always emits alt.
layouts/shortcodes/amazon.html	Adds an alt attribute for Amazon product images with reasonable defaults.
layouts/partials/author.html	Adds an alt attribute to the author thumbnail image.
layouts/_default/summary.html	Adds an alt attribute to summary thumbnail images.
layouts/_default/_markup/render-image.html	Always emits alt for Markdown images and adds a warning when alt is empty.
exampleSite/packages/hugoautogen/package.json	Bumps a few frontend/dev dependency versions.
exampleSite/packages/hugoautogen/hugo_packagemeta.json	Updates the autogen metadata checksum to match dependency changes.
exampleSite/package-lock.json	Updates lockfile entries consistent with dependency bumps.
exampleSite/config/_default/hugo.yaml	Updates Hugo timezone setting to Asia/Tokyo.
docs/guidance/image-accessibility.en.md	Adds a dedicated English image accessibility guide.
docs/guidance/image-accessibility.ja.md	Adds a dedicated Japanese image accessibility guide.
docs/guidance/getting-started.en.md	Links to the new English image accessibility guide.
docs/guidance/getting-started.ja.md	Links to the new Japanese image accessibility guide.

Files not reviewed (1)

• exampleSite/package-lock.json: Generated file