Add bicep description for infra/private-endpoints.bicep

[diberry]

Please provide us with the following information:

This issue is for a: (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

Minimal steps to reproduce

Adding a Bicep description helps me figure out what the file is for and how in is part of the VPN story.

Any log messages given by the failure

Expected/desired behavior

OS and Version?

Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

azd version?

run azd version and copy paste here.

Versions

Mention any other details that might be useful

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Thanks! We'll be in touch soon.

[pamelafox]

Assigning to @mattgotteiner so he can bundle it with other vnet Bicep improvements

[diberry]

@pamelafox @mattgotteiner I found another sample, related to VNETS work, where the user enters the password during azd up and the user name is hard coded.