docs: document Ninja reconfigure target

[ColumbusLabs]

Closes #2409.

Document ninja -C builddir reconfigure in the existing “Building directly with Ninja” section and clarify that it forces Meson reconfiguration without building targets.

Validation:

• verified the documented command against a generated Ninja build; it reconfigured successfully and built no targets
• python3 run_format_tests.py
• added lines remain within the documentation 70-character guideline
• git diff --check

The full Hotdoc build could not start because the local system lacks json-glib-1.0; the changed fenced Markdown and command were checked independently.