feat: add reStructuredText (.rst) support

[tryh4rd-26]

Works on Issue #38

Hello!

this PR adds support for parsing restructuredtext (.rst) documents into anydoc's internal document model so they can render cleanly to github-flavored markdown.

• added Format::Rst to Format enum and mapped .rst extensions

• added rst parser module in src/formats/rst.rs handling:

  • section headings with adornment levels (=, -, ~, etc.)
  • inline formatting: bold (**), italic (*), code (`code` / :literal:), links, inline roles (:strong:, :emphasis:, :sub:, :sup:, :ref:)
  • bullet, numbered, nested, definition, field lists, and line blocks
  • code blocks (:: and .. code-block:: <lang>)
  • grid tables (with merged cell headers) and .. list-table:: directives
  • directives & admonitions (.. note::, .. warning::, .. important::, .. tip::, .. image::, .. container::, etc.)
  • substitutions, footnotes, and citations

• This works on issue feat proposal: Restructured Text #38

i tested locally using standard test suite and dedicated rst integration tests:

cargo test --test rst
cargo test

also verified conversion against test rst samples:

cargo run --example convert -- "doc1.rst"
cargo run --example convert -- "doc2.rst"
cargo run --example convert -- "doc3.rst"

you can find the local test sample files used here:
https://drive.proton.me/urls/Z9V4V2T69C#IxEQKKw1K9sI

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}

[tryh4rd-26]

@tomsideguide