Markdown link replacement in readme causes broken image links

[kzu]

Current code that performs the readme markdown links replacements has some bugs:

1. Full url is expanded to a blog url rather than a raw URL. For display in nuget.org, we must use raw (i.e. https://github.com/devlooped/nugetizer/blob/6f88c3fd7e1c3b161d1364eed73fa8ab37e82455/foo.png should be https://github.com/devlooped/nugetizer/blob/6f88c3fd7e1c3b161d1364eed73fa8ab37e82455/foo.png

2. Links can also have a link tooltip in addition to text and url, such as: [![mccaffers](avatars/mccaffers.png "mccaffers")](https://github.com/mccaffers). We should preserve that if present.

3. We got this link [![torutek](https://raw.githubusercontent.com/devlooped/sponsors/main/.github/avatars/torutek.png "torutek")](https://github.com/torutek) replaced as [![torutek](https://github.com/devlooped/nugetizer/blob/6f88c3fd7e1c3b161d1364eed73fa8ab37e82455/https://raw.githubusercontent.com/devlooped/sponsors/main/.github/avatars/torutek.png "torutek")](https://github.com/torutek)