docs: add TaskEither documentation and update README, changelog, roadmap, index

[Copilot]

Adds full documentation for TaskEither (the last undocumented effect) and updates the README to describe all effects instead of advertising the library as pre-v1.

Changes

• docs/task-either.md — Replaces the WIP stub with a complete page: motivation, basic usage (creating, chaining, error handling, running), and a full API reference for all constructors (taskEither, taskLeft, fromEither, fromTask, tryCatch) and methods (map, mapLeft, flatMap, flatten, zip, ap, tap, tapLeft, match, getOrElse, orElse, run, all)

• docs/index.md — Adds a TaskEither section to the Getting Started page, consistent with the existing Option, Either, Validation, and Task sections

• README.md — Replaces the pre-v1 status block with a concise effects table covering all five types

• CHANGELOG.md — Records the documentation additions under [Next]

• ROADMAP.md — Checks off (doc) TaskEither documentation

TaskEither quick example

import { tryCatch, all } from "ok-fp/taskEither";

const fetchUser = (id: string) =>
  tryCatch(
    () => fetch(`/api/users/${id}`).then((r) => r.json()),
    (err) => `Fetch failed: ${err}`,
  );

// Chain async steps — short-circuits on first error
const greeting = fetchUser("a-001").map((user) => `Hello, ${user.name}!`);

const result = await greeting.run(); // Either<string, string>

// Concurrent requests
await all([fetchUser("a-001"), fetchUser("b-002")]).run(); // Either<string, User[]>

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[pwlmc]

lgtm