feat(docs): add social cards and llms.txt, fix zensical 0.0.72 privacy error - #9009

…y error [skip buildkite]

## Short Summary (TL;DR)

zensical 0.0.72 broke the docs build on every PR. This PR fixes the privacy plugin config, and adds social cards, `llms.txt`, and a "Copy as Markdown" button.

## The Issue

- `zensical build` fails with `Error: privacy enabled must be a boolean`, because `!ENV [ READTHEDOCS, '' ]` gives an empty string when `READTHEDOCS` is unset. Earlier zensical versions ignored the `privacy` plugin, so docs.ddev.com has loaded Google Fonts from Google since the migration to zensical.
- zensical 0.0.67 added native `social` and `llmstxt` plugins. Before, a shared docs link showed no preview image, and AI agents had to scrape HTML.

## How This PR Solves The Issue

Privacy:

- Use `false` as the fallback. Read the Docs sets `READTHEDOCS=True`, which zensical parses as a boolean.
- Fetch only Google Fonts. The theme JS also references unused libraries on unpkg, and those pull in video and Google ad SDKs. That was 6.4 MB from 9 hosts, and one timeout failed a local `--strict` build. With the allow-list, the build fetches 31 font files (896 KB).
- Require zensical 0.0.72, because older versions skip `privacy` without a warning.

Social cards and `llms.txt`:

- `social` makes a 1200x630 card for each page and adds Open Graph and Twitter tags. The card crops its logo to a square, and `logo.svg` is wider than tall. So `docs/social-logo.svg` is a copy of the logo with a square viewBox.
- `llmstxt` writes [`llms.txt`](https://llmstxt.org/), `llms-full.txt`, and an `index.md` next to each page. It also adds `<link rel="describedby">` and `<link rel="alternate" type="text/markdown">` to each page.
- The `content.action.copy` feature shows the button. It needs `llmstxt`, which only exports pages that its `sections` match. So the sections use one wildcard per docs directory, and a new page in an existing directory is covered without config changes.
- A new FAQ entry tells users about these files, because most assistants don't look for `llms.txt` on their own. Its links are relative, so each docs version links to its own files. `.linkspector.yml` ignores them, because they don't exist in `docs/content`.

The cards add about 4 seconds to an uncached build. The first build downloads the card font from Google Fonts into `.cache/plugin/social`.

zensical writes `&` from the page text as `&amp;` in the Markdown copies, and as `&amp;amp;` in `llms.txt` titles. This bug is upstream, and it affects 10 pages.

## Manual Testing Instructions

```bash
scripts/install-dev-tools.sh
make zensical
# Expect "No issues found"
READTHEDOCS=True zensical build --clean --strict
ls site/assets/external
# Expect only fonts.googleapis.com and fonts.gstatic.com
ls site/llms.txt site/llms-full.txt site/users/quickstart/index.md
grep 'og:image' site/users/quickstart/index.html
```

In the Read the Docs preview, open https://ddev--9009.org.readthedocs.build/en/9009/users/quickstart/ and click "Copy as Markdown" next to the edit button. Then open https://ddev--9009.org.readthedocs.build/en/9009/assets/images/social/users/quickstart.png. In https://ddev--9009.org.readthedocs.build/en/9009/users/usage/faq/#can-ai-assistants-read-the-ddev-docs, the `llms.txt` and `llms-full.txt` links must open the files for that version.

## Automated Testing Overview

The docs-check workflow builds with `--strict` and no `READTHEDOCS`, which covers the boolean fix. A section path without a wildcard that names no page also fails that build.

🤖 Developed with assistance from [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>