docs: explain the boot chain, SoftDevice, and the three update paths

[jamesarich]

Summary

Reviewing the docs after PR #14's overhaul: they cover usage well (how to flash, troubleshoot, board list) and give a decent file-by-file architecture breakdown in AGENTS.md, but never explain the underlying concepts someone new to this repo actually needs first:

• That this repo is only the bootloader — the Meshtastic application itself lives entirely in meshtastic/firmware and is built/flashed separately. Nothing said this plainly before.
• How the boot chain actually works (MBR → this bootloader → application), referenced only in passing as "MBR/SoftDevice handoff" in AGENTS.md with no explanation.
• What a SoftDevice even is — referenced constantly (SD_VERSION, S140 6.1.1) but never defined.
• That UF2 drag-and-drop, serial DFU, and BLE OTA DFU are three different transports for the same job, not three unrelated features — the README documents how to use each individually but never says this.

Changes

• New README.md section, How this fits together, right after the intro (before "Boards supported"): the boot chain, the "this repo ≠ the application" split, a table of the three update paths with pointers to where each is already documented in detail, and a short SoftDevice explainer.
• AGENTS.md's "First read" now points at that section first, since the Architecture section's terminology (MBR/SoftDevice handoff, DFU state machine) assumes it.

No functional changes — docs only.

Test plan

• PR title specifically describes the change
• tools/build_all.py (or CI's board matrix) passes — unaffected by a docs-only change, verified green anyway
• Proofread rendered Markdown (table, anchors, section ordering) for correctness
• [N/A] No code, board, or CI config touched

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📥 Commits

Reviewing files that changed from the base of the PR and between 18758d9 and 1d6417f.

📒 Files selected for processing (2)

• AGENTS.md
• README.md

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