docs: streaming saves, memory cards, and new container config keys

[LoneAngelFayt]

Follow-up to #99. The streaming PR (rommapp/romm#3856) added memory card sync, save-state history, and a few new container config keys that the current docs don't cover yet. This fills those gaps.

What changed

docs/using/emulator-streaming.md

• Corrected the emulator count in the work-in-progress note (three, not four; the fourth integration was dropped).
• Rewrote the save states section as "Saves and save states": how manual and autosave slots work, how states are copied into the library on exit, the Resume from state panel, and STREAMING_STATE_HISTORY_LIMIT pruning.
• Added a "Memory cards" section: opt-in memory_card_sync on PS2 and GameCube, the whole-card round trip, and the first-use Import this card / Start fresh prompt.
• Added a warning that playing on the container directly (outside RomM) does not sync back, since a later RomM session loads the library copy over it.
• Documented the new container keys in the setup section: memory_card_sync, library_path, emulator.

docs/reference/configuration-file.md

• Added memory_card_sync, library_path, and emulator rows to the streaming.containers table, with a cross-link to the memory cards section.
• Added a memory_card_sync line to the ps2 example.

Notes

• Grounded against the streaming endpoint code (slot counts, card platforms, config keys, env vars) rather than paraphrasing the PR.
• trunk fmt applied; prose kept in the existing mechanism-first voice.

AI assistance: drafted with the help of an AI coding assistant, then reviewed and verified against the source by me.