doc: add end-to-end tutorial

doc/tutorial.md walks from probe detection to JEDEC ID over EXTEST and
forward-references the BSCAN proxy path. Includes:
- prerequisites and build/launch
- chain scan, FPGA identification, registry lookup
- IR/DR primitive sanity check via the IDCODE register
- SPI JEDEC ID over EXTEST (with the speed caveat)
- recipe to add a new FPGA target
- troubleshooting cheat sheet

README and CLAUDE.md updated to point at it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -81,6 +81,8 @@ bs_explorer> jtag_spi_rd_wr 9F000000
 ```
 
 A minimal example script is provided in `scripts/example_script.txt`.
+A full step-by-step walkthrough (probe detection → JEDEC ID → flashing
+via BSCAN proxy) lives in [`doc/tutorial.md`](doc/tutorial.md).
 
 ## Main commands
 
@@ -126,6 +128,7 @@ modules/
 └── natsort/         Natural-order pin-name sorting
 bsdl_files/          BSDL files for target FPGAs
 scripts/             Example scripts
+doc/                 Tutorial and longer-form docs
 libs/libftd2xx/      Vendored FTDI SDK
 ```