docs: Update release notes for 1.9.0 release

- Table of Contents

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## Version 1.9.0

The 1.9.0 release of MCUboot contains various bug fixes, improves

support on some recent targets, and adds support for devices with a

write alignment larger than 8.

This change introduces a potentially incompatible change to the format

of the image trailer.  If `BOOT_MAX_ALIGN` is kept at 8, the trailer

format does not change.  However, to support larger write alignments,

this value can be increased, which will result in a different magic

number value.  These targets were previously unsupported in MCUboot,

so this change should not affect any existing targets.  The change has

been tested with a `BOOT_MAX_ALIGN` up to 32 bytes.

### About this release

- Add native flash encryption to Espressif targets

- Numerous documentation improvements

- Increase coverage of large images in the simulator

- Add stm32 watchdog support

- Add support for the `mimxrt685_evk` board

- Add support for "partial multi-image booting"

- Add support for clear image generation with encryption capability to

  imgtool

### Security fixes

## Version 1.8.0

The 1.8.0 release of MCUboot contains numerous fixes, and adds support