ci+docs: nginx matrix job and Horde install walkthrough

[ralflang]

Summary

Some loosely-coupled improvements that all help automating and documenting initial installation of a new horde setup.

See also #11

1. CI matrix over apache2 and nginx. install-integration.yml now runs the same install lane twice: once with apache2, once with nginx. Shared steps live once; per-webserver steps gate on matrix.webserver. fail-fast: false so a failure in one flavor doesn't hide a pass in the other.

2. doc/INSTALL_HORDE.md. Human-readable walkthrough of the same flow the CI automates, structured so future pivots (nginx/apache2 today; mysql/mariadb/postgresql later; PHP variant later) slot into a matrix of small recipe sections without disturbing the main description flow.

3. Doc tree cleanup. The nested doc/Horde/Hordectl/ was a PEAR-era artifact

The matrix strategy with if: matrix.webserver == '<flavor>' on the per-flavor steps was the smallest primitive that keeps shared code in one place while letting each branch express its own webserver-configuration idiom.

4. webroot dir vs bundle root dir Previously the CI would fixup the vhost webroot to be the /web/ dir under the root bundle's dir. This is now automatically written correctly.