docs: change the icon of the button showing/hiding the second level of Side Nav

[InnaAtanasova]

Related Issue

Part of #2045

Description

Add JS to the documentation for changing the icon of the button responsible for opening/hiding the second level of the Side navigation

Screenshots

Before:

After:

Please check whether the PR fulfills the following requirements

1. The output matches the design specs
   NA
2. The code follows fundamental-styles code standards and style
   NA
3. Testing

• [na] tested Storybook examples with "CSS Resources" normalize option
• [na] tested Storybook examples with "CSS Resources" unnormalize option
• [na] Verified all styles in IE11
• Updated tests

4. Documentation

• Storybook documentation has been created/updated
• [na] Breaking Changes wiki has been updated in case of breaking changes.

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