docs: mirror source datasheets needed by soundwave pre-BOM sweep

[zackees]

Why

FastLED/soundwave#53 is closing the pre-BOM design sweep. The CH32V203 data sheet and reference manual are already mirrored, but the source documents for the fixed parts below are absent from this repository. Keeping verified local copies makes the electrical limits and BOM decisions reviewable without depending on vendor-site availability.

This is a focused follow-up to #1.

Documents to add

• Microchip MCP1700 data sheet (the host battery rail uses MCP1700T-2502E/TT; needed for output tolerance, dropout, reverse-output behavior, and stability).
• Torex XC6206 series data sheet (used for the host 3.3 V VBUS rail and the soundwake 2.0 V mic rail; capture the applicable P332MR and P202MR variants).
• Microchip MCP6144 / MCP614x family data sheet (soundwake gain/GBW and output swing analysis).
• TI TLV7041 data sheet (soundwake open-drain comparator; required for offset limits across temperature).
• PUI Audio AMM-2742-T-WP-R data sheet (selected top-port IP57 microphone; required for supply, sensitivity, and start-up characterization).
• Microchip MCP73831 data sheet (selected host battery charger).

For each, add a SOURCES.md entry with the official vendor URL, retrieval date, and SHA-256, following wch/mcu/CH32V203/SOURCES.md.

Deliberately deferred

The #53 rail-OR choice is still open, so do not mirror a generic BAT54/BAT60/AO3401A document yet. Add the exact chosen diode/FET data sheet when soundwave#56 selects the topology and BOM SKU.

Refs: FastLED/soundwave#53, #54, #56, #57, #60, #61; zackees/soundwake#16.