docs: add TLS reverse proxy configuration guide (#8673)

* docs: add TLS reverse proxy configuration guide

Add documentation explaining how to use LocalAI behind a TLS
termination reverse proxy (HAProxy, Apache, Nginx).

The documentation covers:
- How LocalAI detects HTTPS via X-Forwarded-Proto header
- Required headers that must be forwarded
- Configuration examples for HAProxy, Apache, and Nginx
- Sub-path serving configuration
- Testing and troubleshooting guide

Fixes: Issue #7176 - Web UI broken behind TLS reverse proxy

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

* docs: remove non-existent --base-url option from sub-path section

---------

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@users.noreply.github.com>