docs: mark unreleased storage backends and mosaic feature in getting-started.md

[JRVector9]

Purpose

getting-started.md documents the storage-cos, storage-azdls, storage-obs, storage-gcs, and mosaic Cargo features as available under paimon = { version = "0.2.0", ... }. None of these exist in the published 0.2.0 crate — they were added on main after the 0.2.0 release.

Reproduced with a scratch crate:

[dependencies]
paimon = { version = "0.2.0", features = ["mosaic"] }

error: failed to select a version for `paimon`.
package `paimon-repro` depends on `paimon` with feature `mosaic` but `paimon` does not have that feature.
help: available features: default, fulltext, storage-all, storage-fs, storage-hdfs, storage-memory, storage-oss, storage-s3, tantivy, tempfile, vortex

Confirmed by diffing the v0.2.0 tag's crates/paimon/Cargo.toml against main: storage-cos, storage-azdls, storage-obs, storage-gcs, and mosaic only exist on main.

Fixes #461

Changes

• Mark storage-cos, storage-azdls, storage-obs, storage-gcs, and mosaic in the features table/section as main-branch-only (not yet in a release).
• Replace their Cargo.toml example with a git dependency instead of a version = "0.2.0" pin. Verified this builds cleanly against main (cargo check succeeds, resolving paimon v0.3.0 from git).