docs: explain templates and fragments together

[roli-lpci]

Closes #918.

The opening of the templates page now explains when to use templates and fragments, then shows them working together: save summarization instructions as a template and provide a document with -f. It links to the YAML form for templates that include fragments.

This also removes the stale opening statement that only one template can be used at a time. Current CLI code, the existing multiple-template test, and the later section on the same page support repeated -t options applied from left to right.

Validation:

• uv run --python 3.11 --with-requirements docs/requirements.txt make -C docs html: build succeeded.
• Executed the example with the repository's local echo model and an isolated LLM_USER_PATH; verified the saved system instructions and document text arrive together in the prompt. No provider call.
• git diff --check: passed.
• The existing template/fragment test run could not complete successfully: current main uses httpx2-pytest while these test modules request httpx_mock. This docs-only diff does not change that fixture setup.

Prepared with AI assistance.