Doc: Converted "installation" group to a regular page.

-regular page doesn't list the group members, allowing extra content
to be listed and gives control on the layout of information
-renamed "Installation" to "Building Qt Sources" as it is a more precise
 name
-Added links to the "Building Qt Sources" page where it makes sense.

Change-Id: I074bc5c17edf2bc096bcfc8f6d102b469929ede0
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Doc: Reworded description of Qt Creator.

-old description is too long

Change-Id: I706a5905c38c36383f2f18cab1620e979c1a86e5
Reviewed-by: Martin Smith <martin.smith@digia.com>
Doc: Fixed WS error in Windows platform page.

-missing space after a full stop.

Change-Id: Ibb81668aa3cd9e67f7289b45ca22947c9cc40be3
Reviewed-by: Martin Smith <martin.smith@digia.com>