Avoid duplication of documentation between extension and wiki

[angelozerr]

Avoid duplication of documentation between extension and wiki

Fixes #331

Signed-off-by: azerr [email protected]

[angelozerr]

This PR provides:

• add a README.md on the docs (home page)
• a vscode XML command to open XML documentation (it opens the README.md)
• add .md for each link to fix navigation with browser (ex : go at https://github.com/redhat-developer/vscode-xml/blob/master/docs/Preferences.md#file-associations and try to click on XSD file associations link, it doesn't work).

Once this PR will be merged, I will remove all content from wiki and set a link to the docs/README.md

[datho7561]

The open documentation command is available in the command palette before the extension starts, but trying to use it before the extension starts results in an error:

I think adding it to the list of extension activation events makes sense.

[angelozerr]

I think adding it to the list of extension activation events makes sense.

fixed