add example placeholders and clean up some signature files

[dsyme]

This adds placeholders for examples to all of FSharp.Core using <example-tbd></example-tbd> and removes some whitespace from some signature files

Best viewed with whitespace changes off https://github.com/dotnet/fsharp/pull/12161/files?diff=unified&w=1

[dsyme]

Strange package download failures in test, I assume nuget was in a huff

2021-09-15T21:52:47.2880423Z   Error Message:
2021-09-15T21:52:47.2881136Z    System.Exception : Evaluation returned 6 errors:
2021-09-15T21:52:47.2882813Z 	input.fsx (2,1)-(2,37) interactive error D:\workspace\_work\1\s\.dotnet\sdk\6.0.100-preview.7.21379.14\NuGet.targets(131,5): error : Failed to download package 'System.Device.Gpio.1.0.0' from 'https://pkgs.dev.azure.com/dnceng/9ee6d478-d288-47f7-aacc-f6e6d082ae6d/_packaging/45bacae2-5efb-47c8-91e5-8ec20c22b4f8/nuget/v3/flat2/system.device.gpio/1.0.0/system.device.gpio.1.0.0.nupkg'. [C:\h\w\B60A09D2\t\nuget\6700--e9aada5b-dd18-47f9-a1ca-16bb44971ef9\Project.fsproj]
2021-09-15T21:52:47.2885687Z 	input.fsx (2,1)-(2,37) interactive error D:\workspace\_work\1\s\.dotnet\sdk\6.0.100-preview.7.21379.14\NuGet.targets(131,5): error : A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. [::ffff:13.107.6.175]:443 (n3kvsblobprodcus335.vsblob.vsassets.io:443) [C:\h\w\B60A09D2\t\nuget\6700--e9aada5b-dd18-47f9-a1ca-16bb44971ef9\Project.fsproj]
2021-09-15T21:52:47.2888504Z 	input.fsx (2,1)-(2,37) interactive error D:\workspace\_work\1\s\.dotnet\sdk\6.0.100-preview.7.21379.14\NuGet.targets(131,5): error :   A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. [::ffff:13.107.6.175]:443 [C:\h\w\B60A09D2\t\nuget\6700--e9aada5b-dd18-47f9-a1ca-16bb44971ef9\Project.fsproj]
2021-09-15T21:52:47.2891918Z 	input.fsx (2,1)-(2,37) interactive error D:\workspace\_work\1\s\.dotnet\sdk\6.0.100-preview.7.21379.14\NuGet.targets(131,5): error : The feed 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json [https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json]' lists package 'System.Device.Gpio.1.0.0' but multiple attempts to download the nupkg have failed. The feed is either invalid or required packages were removed while the current operation was in progress. Verify the package exists on the feed and try again. [C:\h\w\B60A09D2\t\nuget\6700--e9aada5b-dd18-47f9-a1ca-16bb44971ef9\Project.fsproj]

[dsyme]

@TIHan @vzarytovskii @KevinRansom This is ready