Task 10337 Add doc folder

[WilsonZiweiWang]

Since we don't want to read users' bitbake folder, I added a doc folder at the project root. (Currently, the folder contains the files needed by the bitbakeDocScanner parse functions)
I passed the extension path (which points to client/ in our case) in initializationOptions so the server can receive it. The path is then used to construct the doc path in bitbakedocScanner. So after the compilation, the server still knows the correct path (since it is relative to the client folder). A default path is provided to pass the tests. (Or set the path in tests?)

[deribaucourt]

The code is great but I think we should make sure this is the approach we want.
Like task.rst, we must identify a maintainable way to update and package external documentation files.

[deribaucourt]

I tested that the .vsix properly showed hover documentation for variables. However it doesn't for tasks but I don't think we implemented that yet.

[WilsonZiweiWang]

I tested that the .vsix properly showed hover documentation for variables. However it doesn't for tasks but I don't think we implemented that yet.

Yes, we didn't implement it in the onHover, I prefer to do it another PR since I am planning to optimize the bitbakedocscanner which is related to it.

[deribaucourt]

LGTM