added gitignore for dotnet core web app

[zakk616]

Reasons for making this change:

TODO

Links to documentation supporting these rule changes:

TODO

If this is a new template:

• Link to application or project’s homepage: TODO

[salmonslay]

I don't think adding other IDEs like Jetbrains Rider is appropriate here, and the Rider ignored files aren't even correct; the entire .idea should not be ignored according to Jetbrains themselves. https://intellij-support.jetbrains.com/hc/en-us/articles/206544839