Release 0.2.16

What's Changed

• docs: Port Python conceptual doc changes by @jacoblee93 in #603
• docs: Port subgraph how-tos from Python by @jacoblee93 in #604
• docs: update image in multi-agent concepts by @vbarda in #605
• docs: Fix style and links by @jacoblee93 in #606
• feat(langgraph): Add base error class, improve error messages by @jacoblee93 in #601
• feat: Move error reference pages by @jacoblee93 in #607

Full Changelog: 0.2.15...0.2.16