v0.2.0

Release 0.2.0

Version, the `[0.2.0]` CHANGELOG section, and a `TSTP` docs group for the
five modules added: `Tptp.Derivation` with its node and step structs,
and `Tptp.Tstp` with its solution struct.

The README gains a "Derivations and TSTP files" section with one example
of each, and the Function list an eighth entry: reading a derivation is
a thing the library does now, not a note under the SZS reader.

The citation metadata says so too — a release that adds a reader for a
format is one someone may cite for that reader.

`Tptp.bnf_version/0` does not move: the grammar was not regenerated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>