[Custom Errors] Add bandwidth warning for large custom error assets - #33298

Fix the following review findings in PR #33298 (https://github.com/cloudflare/cloudflare-docs/pull/33298).

Before making changes, review each finding and present a brief summary table:
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- If you disagree (e.g. the fix requires disproportionate effort for minimal benefit,
  or the finding is factually incorrect), explain why
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After triaging, follow this order:
1. Post a comment on this PR for any findings you are skipping, with the finding ID and your reasoning.
2. Then commit the fixes for the legitimate findings.

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## Style Guide Review

### Suggestions (1)

#### SG-6fe168da3848 · Passive voice
- **File:** `src/content/docs/rules/custom-errors/index.mdx` line 106
- **Issue:** "a custom error asset is served to every visitor" and "external URLs are fetched and inlined as well" use passive voice; the actor (Cloudflare) is known and used as subject in the preceding clause
- **Fix:** Rewrite in active voice, e.g. "Cloudflare serves a custom error asset to every visitor that triggers the associated rule... Cloudflare inlines referenced resources when fetching the asset, and also fetches and inlines external URLs."