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

The site's own libosrm page (/docs/<version>/libosrm) has, since 2016, pointed C++
embedders at header files on GitHub instead of offering a reference. This adds the
rendered reference to the site so those types are browsable here, and links it from
the front page next to DEMO and DOCUMENTATION.

libosrm-api/ is generated output pinned to osrm-backend commit
e9abd4e0268c692210c4f8e6dcd1d184ff2c8f72 (tip of master, after v26.8.0). Every file
except the README is byte-for-byte the output of tools/api-docs/build.py in the
backend repo, which runs Doxygen over the public headers and renders the XML with
sourcey@3.6.5. Every symbol links back to its exact file and line at that commit.

_og/ is intentionally not committed: Jekyll drops leading-underscore paths on
GitHub Pages and those files are only social-preview images.

Author: antheducation

index.html

@@ -19,7 +19,7 @@
         <div class='col6 margin3 pad4y clearfix'>
           <div class='pad2y title '>Modern C++ routing engine for shortest paths in road networks.</div>
           <div class='center'>
-            <a href='https://map.project-osrm.org/'><input type='button' class='button' value='VIEW DEMO' name='' /></a> <a href='docs/'><input type='button' class='button' value="DOCUMENTATION" name='' /></a>
+            <a href='https://map.project-osrm.org/'><input type='button' class='button' value='VIEW DEMO' name='' /></a> <a href='docs/'><input type='button' class='button' value="DOCUMENTATION" name='' /></a> <a href='libosrm-api/'><input type='button' class='button' value="LIBOSRM C++ API" name='' /></a>
             <p class='pad2x'><a class="github-button" href="https://github.com/Project-OSRM/osrm-backend" data-icon="octicon-mark-github" data-style="mega" aria-label="View Project-OSRM/osrm-backend on GitHub">View on GitHub</a></p>
           </div>
         </div>