Improve scriv create fragment template

[ncoghlan]

The default scriv fragment is quite noisy, and doesn't establish conventions for particular fragments.

Default templates (as modified by the open scriv PR at nedbat/scriv#134).

Markdown:

<!--
A new scriv changelog fragment.

Uncomment the section that is right (remove the HTML comment wrapper).
For top level release notes, leave all the headers commented out.
-->

{% for cat in config.categories -%}
<!--
### {{ cat }}

- A bullet item for the {{ cat }} category.

-->
{% endfor -%}

ReST:

.. A new scriv changelog fragment.
{% if config.categories -%}
..
.. Uncomment the section that is right (remove the leading dots).
.. For top level release notes, leave all the headers commented out.
..
{% for cat in config.categories -%}
.. {{ cat }}
.. {{ config.rst_header_chars[1] * (cat|length) }}
..
.. - A bullet item for the {{ cat }} category.
..
{% endfor -%}
{% else %}
- A bullet item for this fragment. EDIT ME!
{% endif -%}

[ncoghlan]

Initial thoughts for an update is to just include one representative example, and edit
the category heading in place, instead of having lots of duplicated entries to be deleted.

Markdown:

<!--
Defined changelog categories:
{% for cat in config.categories -%}
  - {{ cat }}
{% endfor -%}

Replace "Category" below with the relevant category name.
For top level release notes, delete the category header entirely.
Use the ReST format for notes that need to refer to issue numbers.

Deleting this header after editing is recommended.
-->

### Category

- Release note details.

ReST:

.. Defined changelog categories:
{% for cat in config.categories -%}
..   - {{ cat }}
{% endfor -%}
..
.. Replace "Category" below with the relevant category name.
.. Adjust the heading underline to match the chosen category name.
.. For top level release notes, delete the category header entirely.
.. Update the referenced issue number as appropriate.
.. "resolved" works for most categories, but replace the verb as necessary.
.. Use `:pr:`NN` to refer to pull requests. Other Sphinx roles also work here.
..
.. Deleting this header after editing is recommended.

Category
--------

- Release note details (resolved in :issue:`NN`).

[ncoghlan]

Defining a markdown fragment template isn't necessary, as I misunderstood the granularity of scriv's format option (you can't mix and match fragment formats - any given project must exclusively use either ReST fragments or Markdown fragments).

(I want to use ReST so the :issue:`NN` and :pr:`NN` semantic references work).