chore: add short description about imu in sensing architecture

[TaikiYamada4]

Description

I found that there is a lack of description about IMU messages in the sensing architecture so I added a simple one.

Related discussion: https://github.com/orgs/autowarefoundation/discussions/5064#discussioncomment-10251170

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[github-actions]

Documentation URL: https://autowarefoundation.github.io/autoware-documentation/pr-607/
Modified URLs:

• https://autowarefoundation.github.io/autoware-documentation/pr-607/design/autoware-architecture/sensing/

[TaikiYamada4]

@knzo25 @drwnz

Excuse me for interrupting.
I chose you as reviewers based on the previous PRs.

I believe this PR hurts nothing, but if there is someone who suits the role better please add them to this PR.

[Shin-kyoto]

Thanks!! LGTM.

I checked this PR and I confirmed that this is good to merge.