docs: (core) Update popover's usage of side nav, add important files of popover to public_api.ts

[JKMarkowski]

Please provide a link to the associated issue.

fixes: #1644

Please provide a brief summary of this pull request.

There is new side navigation component usage on popover's last example.
Also during implementation I noticed that there are some files missing in popover's public_api,ts

Please check whether the PR fulfills the following requirements

• the commit message follows the guideline:
  https://github.com/SAP/fundamental-ngx/blob/master/CONTRIBUTING.md
• tests for the changes that have been done

Documentation checklist:

• update README.md
• Breaking Changes Wiki
• Documentation Examples

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[InnaAtanasova]

Maybe it needs rebasing but I still see the third level of navigation in the examples. Also, the padding's not correct in the example with only text.
When there's only text, the second level items should have 2rem left padding. The next level adds +1rem padding and so on. In NGX the second level items have 2.5rem left padding.

[JKMarkowski]

Hi @InnaAtanasova I think this issue occurs, cause of netlify's cache issues. I think we can merge it.

[InnaAtanasova]

It still has the issue with the paddings.
NGX:

Styles:

Second level items should not have icons. You need to remove the icons from the example:

It should look like this (also the paddings):

[JKMarkowski]

@InnaAtanasova Changes fixing the padding are included now. I also removed 2nd level icons.

[InnaAtanasova]

The text and the icon are not aligned properly:

See a comparison with Styles:

[stefanoScalzo]

The focus is going outside of the side nav

[JKMarkowski]

Hi @InnaAtanasova , @stefanoScalzo I think these comments went too far. This PR was only about fixing side-nav usage on popover component.
@InnaAtanasova For wrong alignment I'm not able to reproduce such a error,
@stefanoScalzo For Focus it's about styles.