Add instructions for flashing direct via esptool

In case anyone has issues with the built-in updating, you can flash directly with `esptool.py`:

```
esptool.py write_flash \
0x0 K-Touch_v1.1.0_bootloader.bin \
0x8000 K-Touch_v1.1.0_partition.bin \
0x10000 K-Touch_v1.1.0_firmware.bin \
0x910000 K-Touch_v1.1.0_product.img
```

Would be nice to have this added to instructions somewhere, especially because the built-in wifi on these chips is very underpowered; it can be much faster to download and flash via a computer than to do it OTA if you have a spotty wifi signal.

Locate the existing firmware update instructions and determine where direct flashing guidance belongs. Document the provided esptool.py command and its image files and offsets, then verify that the instructions clearly explain the computer-based flashing path alongside the built-in update method.

Written by the indexing model from the issue text.