Add table of removed extensions

Add a table of the extensions that were incorporated into SYCL 2020.

Author: gmlueck

sycl/doc/extensions/removed/README.md

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+This directory contains an archive of old DPC++ extensions which are no longer
+implemented.
+
+Normally, a supported extension is first marked "deprecated", and the compiler
+raises a warning for applications that use it.  After the deprecation period,
+support for the extension may be removed, and the specification for the
+extension is moved to this directory for reference.
+
+Experimental extensions may change or be removed without any deprecation
+period.  Since we do not expect production code to use experimental extensions,
+we do not archive their specifications when they are changed or removed.
+Likewise, we do not archive "proposed" extension specifications if we later
+decide not to implement them.
+
+Note that the following extension specifications have been removed because
+their features have been incorporated into the core
+[SYCL 2020 specification][1].  Please see that document for the most accurate
+description of these features.
+
+[1]: <https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html>
+
+| Extension                                     | Description                                                 |
+|-----------------------------------------------|-------------------------------------------------------------|
+|SYCL\_INTEL\_bitcast                           | Adds `sycl::bit_cast`                                       |
+|SYCL\_INTEL\_device\_specific\_kernel\_queries | Adds `info::kernel_device_specific` queries                 |
+|SYCL\_INTEL\_attribute\_style                  | Changes position of C++ attributes                          |
+|Queue Order Properties                         | Adds `property::queue::in_order`                            |
+|SYCL\_INTEL\_parallel\_for\_simplification     | Makes calls to `parallel_for` less verbose                  |
+|Queue Shortcuts                                | Adds shortcut functions to `queue`                          |
+|SYCL\_INTEL\_relax\_standard\_layout           | Drops standard layout requirement for data in buffers, etc. |
+|Unified Shared Memory                          | Adds new unified shared memory APIs                         |
+|SYCL\_INTEL\_unnamed\_kernel\_lambda           | Makes kernel type-names optional                            |
+|SYCL\_INTEL\_deduction\_guides                 | Simplifies SYCL object construction by using C++ CTAD       |