Add automated checks for shared content quality

## Context

These TODOs were identified during a review of the `content-editing` skill and represent automation opportunities to improve shared content quality and consistency.

## Tasks

### 1. Validate `alt_links` for shared content with differing paths

Add an automated check to validate that `alt_links` frontmatter is present when shared content paths differ across products.

**Example:** If a page exists at `/influxdb3/core/admin/tokens/` but `/influxdb3/cloud-serverless/reference/tokens/`, the frontmatter files should include `alt_links` to cross-reference them.

### 2. Check for product-specific URL patterns in shared content

Add a check for product-specific URL patterns that shouldn't appear in shared content.

**Example:** Cloud Serverless uses `/reference/regions` for region URLs, but Cloud Dedicated/Clustered don't have this page (cluster URLs come from account setup). Shared content shouldn't hardcode these product-specific paths.

### 3. Validate resource term links

Add a check/helper to ensure product resource terms (tokens, databases, buckets) link to proper admin pages using the `/influxdb3/version/admin/` pattern.

**Example:** When documentation mentions "database token", it should link to the appropriate `/influxdb3/version/admin/tokens/` page.

### 4. Improve `code-placeholder-key` workflow

The current `docs placeholders` command adds `placeholders` attributes to code blocks but doesn't generate the "Replace the following:" lists with `{{% code-placeholder-key %}}` shortcodes.

**Options:**
- Improve automation to generate these replacement lists automatically
- Simplify by removing `code-placeholder-key` shortcode if the `placeholders` attribute alone is sufficient

## Related

- `.claude/skills/content-editing/SKILL.md` - Content editing workflow skill
- `scripts/docs-cli/docs-cli.js` - docs CLI implementation

Start by reading .claude/skills/content-editing/SKILL.md and scripts/docs-cli/docs-cli.js, then inspect the current docs placeholders command. Map how shared-content frontmatter, product-specific URLs, resource-term links, and code-placeholder-key are handled. Done means the agreed checks or workflow improvements are automated and verified against representative documentation cases.

Written by the indexing model from the issue text.