docs(specifiers): add is_unsatisfiable() usage example

[r266-tech]

Fixes #1165

PR #1119 added SpecifierSet.is_unsatisfiable() but the usage examples in docs/specifiers.rst don't demonstrate it. Users scanning the Usage section have no way to discover this method without reading the full API reference.

Added a doctest example showing is_unsatisfiable() on both an unsatisfiable set (>=2.0,<1.0 → True) and a satisfiable one (>=1.0,<2.0 → False), placed after the existing filter() example.

[henryiii]

@notatallshaw does this look fine, or would you like something else? I could see something like (SpecifierSet(">=3.9,<4.0") & SpecifierSet("==3.12.*").is_unsatisfiable() being closer to what someone might do (the first one would likely come from requires-python).

[notatallshaw]

Yeah, I would expect people would be compounding specifiers, for whatever reason.

We should encourage & because it's faster than recreating a new specifier via string joining, so let's add at least one example using that.

[r266-tech]

Added a compound & example per the review — shows (SpecifierSet(">=3.9,<4.0") & SpecifierSet("==3.12.*")).is_unsatisfiable() (False) and (SpecifierSet(">=3.13,<4.0") & SpecifierSet("==3.12.*")).is_unsatisfiable() (True) — closer to the real-world pattern of combining requires-python with another constraint via &. Let me know if you'd prefer different pairings or a different placement.