Update Forge Standard Library docs

After `1.2.0` release, Forge Standard Library V1 should be stable.

The documentation hasn’t been updated for versions `1.0.0` and later - most notably, the structure is different and new features have been added:

- List components (e.g. `StdStorage`, `StdChains`)
- Describe features (e.g. `isFork`, `getChain`)

See [commit history](https://github.com/foundry-rs/forge-std/commits/master).

Start by reviewing the Forge Standard Library documentation and the linked forge-std commit history, focusing on changes from versions 1.0.0 onward. Update the documentation to match the new structure, list components such as StdStorage and StdChains, and describe features such as isFork and getChain; done means the V1 documentation reflects the stable 1.2.0 release.

Written by the indexing model from the issue text.