Modify Evidence Preview to render only markdown documents in `/pages/` folder

[RandomFractals]

and use the built-in markdown.showPreview command from #29 to show preview of all other .md files that are not inside Evidence app /pages/ folder.

• Check evidence.hasProject context value to determine if the open workspace contains an Evidence project folder
• Also, check if the requested preview page Uri contains /page/ in its path. Assume those markdown documents are Evidence markdown documents
• Render Evidence markdown documents by starting Evidence dev server and opening the requested page preview in the built-in simple browser webview
• Render all other .md markdown documents using the built-in vscode markdown language features and markdown.ShowPreview command restored in #29
• This should be resolved before we ship v1.1.0 (#65) for proper default markdown documents preview handling

[RandomFractals]

Updated Evidence and regular markdown documents Preview handling, based on .md document location, Evidence project /pages/ folder presence, etc.

[archiewood]

On review, I think this is the issue I've noticed in #70

[RandomFractals]

That is correct. I will resolve it in #70 as it has more details capturing server start with / app path scenario.