docs: add EN/PL documentation catalog

[z4nr34l]

Adds a docs/ directory holding the long-form documentation for the base image, in English and Polish. There is no docs/ in the repo today; this is additive only. No Dockerfile, setup.sh, wizard.sh, src/, README or CI files are touched.

What the directory is

README.md stays the landing page. docs/ is where behaviour is documented in full: the setup sequence step by step, every feature option, and the failure modes that only turn up in real use. Keeping it in this repo means it versions with the Dockerfile, setup.sh and src/ it describes.

The same directory is the source for the rendered docs at zanreal.com/docs - the site pulls these files, so there is no second copy to keep in sync. docs/README.md explains that arrangement for anyone who opens the directory cold.

Contents

Three pages per locale:

Page	Covers
index.{en,pl}.mdx	What the image is, quick start, what it ships and deliberately does not, the setup wizard, headless/CI use
features.{en,pl}.mdx	The ten published features, feature vs wizard, install ordering, the Traefik reverse proxy
configuration.{en,pl}.mdx	The nine setup.sh steps, cache isolation, extending the image, image tags

Both locales are written natively rather than translated, so they differ by design - the English configuration page walks the setup sequence as numbered steps, while the Polish one opens with a nine-point overview and then breaks out only the four steps that have traps in them.

Verified against the repository

This content had not been reviewed before, so the factual claims were checked against the source rather than taken on trust:

• the ten features and their versions (bun 1.0.0 through traefik 1.3.2, uv 1.0.1) against each src/*/devcontainer-feature.json
• the DEVCONTAINER_TOOLS identifiers against the case arms in wizard.sh
• the marker files ~/.devcontainer-selections, ~/.devcontainer-wizard-done and /tmp/devcontainer-wizard.log
• the Tinybird install line, including the uv self-bootstrap when uv is absent
• the traefik:v3.7 image tag in src/traefik/install.sh

All matched. No corrections to the README were needed.

Fixed while preparing this

• Two Polish pages had frontmatter that would not parse. configuration.pl.mdx and features.pl.mdx had unquoted description values containing a colon, which YAML reads as a nested mapping and rejects with YAMLParseError: Nested mappings are not allowed in compact mappings. Both are now quoted; all six pages parse.
• A calque in the Polish features table. The feature-identifier column was headed Odwołanie, a word-for-word rendering of "Reference" that means a citation or an appeal in Polish, not an identifier. Changed to Identyfikator. Also softened powtarzalne budowania co do bajta to buildy powtarzalne co do bajta, which is how the phrase is actually said.
• No em dashes anywhere in the added files.

Formatting choices

The content was authored for our fumadocs site and has been adapted for a repo where GitHub is at least as likely a reader as the docs site:

• Links between pages are relative (./features.en.mdx), so they resolve in both places. The link to examples/devcontainer.json stays an absolute GitHub URL, since a relative path out of docs/ would break on the docs site.
• fumadocs-only MDX was converted, not stripped. <Callout> and <Callout type="warn"> became > [!NOTE] and > [!WARNING], which GitHub renders natively as styled callouts and which fumadocs also understands - this is the one case where the portable form is as good as the original. <Cards> became a Markdown list, <Steps>/<Step> became numbered headings, and the // [!code highlight] annotations were dropped. All of these would otherwise show up as literal text on GitHub.
• ```jsonc title="..." blocks were kept as-is. GitHub ignores the meta string and still highlights the block, while the docs site uses it to label the file - so there is nothing to gain by removing it.
• meta.json / meta.pl.json are kept. They carry page order and section title for the docs site, are inert on GitHub, and nothing else in the repo depends on them.

CONTRIBUTING.md covers local image builds, feature testing and the tag-driven release flow; none of it applies to a docs-only change, and nothing here conflicts with it.

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