Publish the generated libosrm C++ API reference at /libosrm-api/

[antheducation]

What this is

The rendered C++ API reference for libosrm, published at /libosrm-api/, plus a
LIBOSRM C++ API button on the front page next to DEMO and DOCUMENTATION.

Companion PR in the backend repo: Project-OSRM/osrm-backend#7677, which
adds the generator (tools/api-docs/) and points docs/libosrm.md and the README here.

Why

/docs/<version>/libosrm documents the C++ library by linking each type to its header file on
GitHub — "a short description of what the types do and where to find documentation on it". The
HTTP and Node.js APIs both have real reference pages here; the C++ one never has. This fills
that gap in the place a reader is already looking.

What is in the diff

• libosrm-api/ — 64 generated files: a landing page, an api/ index and 56 symbol pages,
  plus sourcey.css, sourcey.js, a search index, a sitemap and llms.txt / llms-full.txt.
  Static HTML, no build step, no runtime dependencies. 421 symbols, 355 links back to the exact header line each was read from at
  osrm-backend commit e9abd4e0268c692210c4f8e6dcd1d184ff2c8f72 (tip of master, after
  v26.8.0).
• libosrm-api/README.md — states that the directory is generated and gives the one command
  that regenerates it.
• index.html — one added button.

Everything except that README is byte-for-byte the output of
python tools/api-docs/build.py in the backend repo at the pinned commit, so it can be
regenerated and diffed rather than trusted.

Two things worth knowing

• _og/ is deliberately not committed. Sourcey also emits social-preview images under
  _og/; Jekyll drops leading-underscore paths on GitHub Pages, so they would 404. Excluding
  them costs only the og:image previews and keeps the diff ~3.7 MB smaller. If you would
  rather have them, the fix is an include: ["_og"] line in a _config.yml, and I will add it.
• This repository already stores built documentation for every release from v5.5.0 to
  v26.6.1 under docs/, so committing generated output here follows the existing convention
  rather than introducing a new one. I put the reference at the top level rather than under
  docs/<version>/ because it is pinned to a commit rather than to a release; if you would
  prefer it versioned alongside the VitePress builds, say where and I will move it.

Happy to adjust the button wording, the path, or anything else.

[antheducation]

Hi! A gentle ping on this one in case it slipped past — no rush if it's just queued.

Happy to adapt the approach to whatever fits the site's conventions best. In particular, if you'd rather not carry ~3.8 MB of generated HTML in the repository, I can rework this so the pages are produced by a build step instead (the regenerate-and-diff command in libosrm-api/README.md already does the generation deterministically). Also glad to trim the page set or adjust the styling if anything feels off.

Thanks for your time, and for maintaining OSRM!