Adds a frozen ExecConfig dataclass mirroring testcontainers-java's ExecConfig
(command, user, environment, workdir) plus privileged. It is a deliberately
backend-agnostic carrier: only command is required and every other field
defaults to None/False, so it never encodes docker-py's conventions.

All docker-py coercion is confined to the pure _exec_run_kwargs() seam at the
I/O boundary -- the empty-string user sentinel, str(workdir) so pathlib.Path
is accepted, and verbatim command forwarding (docker-py does its own shlex
tokenization). exec() now accepts str | list[str] | ExecConfig and funnels all
three through that single path.

The config->kwargs logic and the exec() normalization branch are covered by
hermetic unit tests (no daemon required); behavioral verification against a
real container follows separately.

Exercises the new fields against a real alpine container: workdir (str and
pathlib.Path) via pwd, environment via env, user via whoami (root vs nobody),
and privileged by reading CapEff from procfs -- a privileged exec gets the
full capability set, so its effective mask strictly exceeds the default exec's
even on an unprivileged container.

Rewrites the executing-commands guide around the real surface: exec() taking
str | list[str] | ExecConfig, the bytes-valued result, and the user /
environment / workdir / privileged options via ExecConfig. Adds the
str-is-tokenized-not-shell-interpreted caveat that the old environment example
silently got wrong (echo $VAR never expanded).

Also removes sections that documented APIs the library never had:
GenericContainer (the generic container is DockerContainer), exec_interactive,
and the timeout= / tty= exec parameters. The TTY section is gone entirely; the
interactive and timeout sections had no real ExecConfig equivalent to map to.