Add icon for truenas_ce core integration

[kayl-codes]

Proposed change

Adds the icon for the upcoming truenas_ce core integration.
This integration is currently available as the HACS custom integration truenas_ce :
https://github.com/kayl-codes/homeassistant-truenas
It is being prepared for submission to Home Assistant Core under the same truenas_ce domain,
this avoids a conflict with the legacy, unrelated custom_integrations/truenas entry already in this repo,
and preserves entity/config continuity for existing HACS users.
This PR is opened ahead of the documentation and core pull requests,
since both of those are expected to link back to this one.

Only icon.png/icon@2x.png are included, no
logo.png/logo@2x.png — the brand's logo and icon are the same square
image, and per the Brands guidelines the icon is used as the logo fallback
in that case.

Type of change

• Add a new logo or icon for a new core integration
• Add a missing icon or logo for an existing core integration
• Replace an existing icon or logo with a higher quality version
• Replace an existing icon or logo after a branding change
• Removing an icon or logo

Additional information

• This PR fixes or closes issue: n/a
• Link to code base pull request: not yet opened (planned as a follow-up to this PR)
• Link to documentation pull request: not yet opened (planned as a follow-up to this PR)
• Link to integration documentation on our website: not yet published

Checklist

• The added/replaced image(s) are PNG
• Icon image size is 256x256px (icon.png)
• hDPI icon image size is 512x512px for (icon@2x.png)
• Logo image size has min 128px, but max 256px, on the shortest side (logo.png)
• hDPI logo image size has min 256px, but max 512px, on the shortest side (logo@2x.png)