Fix link syntax in User Guide (#15539)
Summary of the issue:
Square brackets appear around an URL in the User Guide; probably an incorrect t2t syntax:
For an introductory tutorial, please refer to [https://docs.python.org/3.7/howto/regex.html].
Description of user facing changes
Modified with a full labeled link; the raw URL is now not visible anymore.
While at it and since we now have NVDA+K, I have replaced other raw URLs of the User Guide by full links with a visible label and no visible raw URL anymore. It makes the User Guide more smoothly readable.
Also, links pointing to dessci.com now redirect to wiris.com. Thus, I have replaced links pointing to dessci.com with links targeting an equivalent resource on wiris.com.
Description of development approach
Writing in t2t syntax.
