Readme links to several 404's

[church365]

Describe the bug

The link to https://github.com/microsoftgraph/msgraph-sdk-dotnet/blob/dev/docs/overview.md returns a 404. Several of the other associated documentation links also fail.

Expected behavior

Expect published links to actually link to something useful and relevant.

How to reproduce

Click the link.

SDK Version

No response

Latest version known to work for scenario above?

No response

Known Workarounds

No response

Debug output

Click to expand log

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### Configuration

_No response_

### Other information

_No response_

[andrueastman]

Apologies for this @church365

This is a sideffect of the branch alignement work done recently to rename the default branch to main. Authored #2655 to resolve this.