add cosign verify images doc

[zishen]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
doc for cosign verify images.

Which issue(s) this PR fixes:
Fixes #379 #448

Refer to karmada-io/karmada#3435

Special notes for your reviewer:
@liangyuanpeng @RainbowMango @XiShanYongYe-Chang

[karmada-bot]

Welcome @zishen! It looks like this is your first PR to karmada-io/website 🎉

[RainbowMango]

Note that images is only one kind of artifacts, we are going to add docs for binary, charts and so on.
So, please reserve the room for them.
Maybe we can organize the content as :

# Overview

## Images
### Prerequisites
### Verify image signature
#### Verify image with cosign CLI
#### Verify image with admission controller

## Binaries
## Charts

[zishen]

Note that images is only one kind of artifacts, we are going to add docs for binary, charts and so on. So, please reserve the room for them. Maybe we can organize the content as :

# Overview

## Images
### Prerequisites
### Verify image signature
#### Verify image with cosign CLI
#### Verify image with admission controller

## Binaries
## Charts

good proposal

[RainbowMango]

/lgtm
/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [RainbowMango]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[liangyuanpeng]

Sorry for my late and thanks for your work! @zishen