[ty] Remove redundant methods from the docstrings interface (astral-sh#29602)

## Summary

Consolidate Markdown and plain-text docstring rendering behind the
[`Docstring::render`](https://github.com/astral-sh/ruff/blob/10e360013c389f14697435087b60fb8708626652/crates/ty_ide/src/docstring.rs#L30)
interface. This removes the redundant format-specific methods and
provides a single interface to which we can attach future rendering
options (such as the allowed HTML tags that will be necessary for
astral-sh/ty#4688).

## Test Plan

This is a behaviour-preserving refactor that relies on existing tests.

Author: lerebear