Rename solution, readme and icon files

[Nirmal4G]

Changes

Rename build folder to eng:

• This is a standard build infra directory used in official dotnet projects.
• Rename to eng folder is done early here to get the build folder used for common output paths.

Rename NuGet Icon to Icon.png:

• This is no longer used as a public reference point for NuGet icon URL.
• Also, Icon URL is deprecated. Hence, it's safe to change.

Normalize casing for ReadMe.md:

• Repository information files such as ReadMe, License, etc... are only UPPER_CASE if they are without an extension. With extension, the casing becomes PascalCase or Kebab-Case. The primary reason is attention to the presentation of file names.
• Do Kebab-Case when a phrase is presented. E.g., Code-of-Conduct.md.

Rename solution file to CommunityToolkit.sln:

• The dotnet seems implied and also doesn't stand out in the file list because of the lower casing and d char.
• Spaces are a main issue when doing automation (like using *.sln in build scripts and in URLs it adds %20).

Misc Changes

• Move toolkit.snk file to build sub-directory.
• Remove un-needed and deleted files from solution.
• Update Git Ignore entries to latest from upstream.
• Rename MSBuild props and targets to shorter names.
• Indent text in ThirdPartyNotices.txt with spaces instead.

PR Checklist

• [x] Created a separate branch (other than main/master) in your fork
• [x] Based off latest main branch of toolkit
• [x] Pull Request doesn't include merge commits
• [x] Header has been added to all new source files
• [x] Contains NO breaking changes
• [x] Code follows all style conventions

Other information

• Please Rebase merge if possible.
• Please update the default message option to Default to pull request title and description in the Repository Pull Request settings to have a better commit message instead of Merge pull request #xxxx from repo/branch generic message.

[Nirmal4G]

These changes are separated from #85, which is becoming too big with the recent refactors. So, I'm extracting some of the changes into separate PRs.

[Nirmal4G]

Updated in response to #486

[Nirmal4G]

Updated in response to #507 and #510

[Nirmal4G]

New Changes since last Review

Rename build folder to eng:

• This is a standard build infra directory used in official dotnet projects.
• Rename to eng folder is done early here to get the build folder used for common output paths.

Misc Changes

• Rename MSBuild props and targets to shorter names.

This change is extracted early from patch #96. That PR itself is not yet updated as it depends on changes from this patch and patch #85.