Add documentation for IDE0270

[mavasani]

IDE0270	https://learn.microsoft.com/dotnet/fundamentals/code-analysis/style-rules/ide0270	Use coalesce expression

Steps to follow to add documentation: https://github.com/dotnet/roslyn/blob/main/docs/contributing/Documentation%20for%20IDE%20CodeStyle%20analyzers.md

[Youssef1313]

NOTE: I think this should be documented the same page as https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0029-ide0030

If we chose that, redirections need to be modified:

1. https://github.com/dotnet/docs/blob/43a1ae2b196bf5a49253d339c04f64d0dc7fa0c4/.openpublishing.redirection.fundamentals.json#L63-L70 redirect_urls should point to ide0029-ide0030-ide0270
2. A new redirection is needed from ide0029-ide0030 to ide0029-ide0030-ide0270
3. A new redirection is needed from ide0270 to ide0029-ide0030-ide0270

[CyrusNajmabadi]

Dupe of dotnet/docs#32876

[gewarren]

Fixed by dotnet/docs#36394.