Support alternate examples in README template

[malandis]

Some clients, especially python, have multiple examples that a prospective user should know about. Some of these are version specific, so they cannot be rolled up into a single large example. Because of these, the README generator should support an "alternate example" section. This should support both a header/snippet as well as the code location, eg

"Non-pattern matching example (python <3.10)", "./examples/prepy310/example.py"
"AsyncIO client example (python <3.10)", "./examples/prepy310/example_async.py"

See momentohq/client-sdk-python#219