site: add a page for the .ttrs document source format, plus robots.txt and sitemap.xml

[transitrix]

What this adds

/ttrs/ and /de/ttrs/ — a canonical page for the document source format, for a reader arriving cold from a search for the extension. It covers what a source file contains (YAML header plus prose with {{ … }} directives), how <basename>.<kind>.ttrs is read — the middle segment is the document kind, never a notation — the directive language with its inline and block forms, the committed conformance fragment, the six reference states an implementation must keep distinct, and the strict/lenient profile pair. It links the normative specification, the reference implementation and the conformance fixture.

llms.txt — it stated that every source file carries *.<notation>.transitrix.yaml, which is no longer the whole picture. It now states both canonical extensions, records that which one a notation takes is a property of that notation, and adds the new page to the canonical-reference list.

sitemap.xml and robots.txt — neither existed, so the sitemap URL 404'd. The sitemap lists all fourteen published pages with their language alternates; robots.txt references it and deliberately leaves the /install/go/ redirects crawlable, since they carry their own noindex.

Scope decisions

• The page describes only what is shipped — the format, the reference implementation, the conformance fixture. Nothing beyond that is implied.
• A German mirror was added because every other section on this site has one and the language switcher would otherwise point at a 404.
• One sentence was added to each home page and each DSL-examples page so the new page is reachable by navigation, not only by search.

Verified, not assumed

• Every external link on both new pages returns 200; all are blob/ URLs, not tree/.
• Every internal target exists in the tree.
• sitemap.xml parses as well-formed XML and its fourteen locations match the fourteen published pages one to one.
• Tag balance checked on both new pages; every code block is fully entity-escaped.
• The repository's public-surface guard patterns were run locally over every changed and added file — clean.

Work item: transitrix-hq#69.