doc: a better order for the "adding Vulkan extension" guide.

It makes most sense to extend the Extensions namespace first.

doc/developers.dox

@@ -729,44 +729,45 @@ unless it doesn't affect public API at all.
 
 @section developers-vk-extensions Checklist for adding / removing Vulkan versions and extensions
 
-1.  Install [flextGL](https://github.com/mosra/flextgl)
-2.  Go to `src/MagnumExternal/Vulkan/`:
-    -   Update `extensions.txt` (bump a version or add/remove extensions)
+1. Add new extensions to `src/Magnum/Vk/Extensions.h`
+    -   there's a separate list for instance and device extensions, ensure each
+        in the right list
+    -   order them by extension ID that is mentioned on every extension spec
+        page
+    -   update the numbering to stay monotonic and unique, round up start index
+        of next section to nearest ten to make the updates bearable
+    -   in case there's a lot of new extensions,
+        @cpp Implementation::InstanceExtensionCount @ce /
+        @cpp Implementation::DeviceExtensionCount @ce might needed to be
+        increased
+2.  Add them alphabetically ordered to the correct list in
+    `src/Magnum/Vk/Extensions.cpp`
+3.  Update existing extensions with version in which they become core (last
+    parameter of the `_extension()` macro)
+4.  Get [flextGL](https://github.com/mosra/flextgl) and go to
+    `src/MagnumExternal/Vulkan/`:
+    -   Update `extensions.txt` (bump a version or add/remove extensions),
+        in the same order as `Extensions.h`
     -   Run `./update-flexgl.sh` to update everything
-3.  Check @cb{.sh} git diff @ce for suspicious changes and whitespace-at-EOL
-4.  For every new added function and `CreateInfo` structure, add an entry to
+5.  Check @cb{.sh} git diff @ce for suspicious changes and whitespace-at-EOL
+6.  For every new added function and structure, add an entry to
     `doc/vulkan-mapping.dox`
-5.  For every new `*Feature` structure, expand the @ref Vk::DeviceFeatures
+7.  For every new `*Feature` structure, expand the @ref Vk::DeviceFeatures
     enum according to @ref developers-vk-features
-6.  Add a table listing the new version and all new extensions in it to
+8.  Add a table listing the new version and all new extensions in it to
     `doc/vulkan-support.dox` (take a list of them from the changelog in the
     official spec PDF). Some extensions might be already present in the general
     extension list, move them out of there.
-7.  Add a new `requires-vkXY` page with @c \@m_footernavigation to
+9.  Add a new `requires-vkXY` page with @c \@m_footernavigation to
     `doc/vulkan-support.dox`, mention it as a @c \@subpage at a correct
     position in the list
-8.  Add a new `requires-vkXY` alias to `Doxyfile`, `Doxyfile-mcss` and
+10. Add a new `requires-vkXY` alias to `Doxyfile`, `Doxyfile-mcss` and
     `Doxyfile-public`, copypaste it from existing and change the numbers
-9.  Add new version enum value:
+11. Add new version enum value:
     -   to `src/Magnum/Vk/Version.h`
     -   to the list in `src/Magnum/Vk/vk-info.cpp`
     -   to @ref Vk::InstanceExtension::extensions() and
         @ref Vk::Extension::extensions() in `src/Magnum/Vk/Extensions.cpp`
-10. Add new extensions to `src/Magnum/Vk/Extensions.h`
-    -   there's a separate list for instance and device extensions, ensure each
-        in the right list
-    -   order them by extension ID that is mentioned on every extension spec
-        page
-    -   update the numbering to stay monotonic and unique, round up start index
-        of next section to nearest ten to make the updates bearable
-    -   in case there's a lot of new extensions,
-        @cpp Implementation::InstanceExtensionCount @ce /
-        @cpp Implementation::DeviceExtensionCount @ce might needed to be
-        increased
-11. Add them alphabetically ordered to the correct list in
-    `src/Magnum/Vk/Extensions.cpp`
-12. Update existing extensions with version in which they become core (last
-    parameter of the `_extension()` macro)
 
 In order to remove Vulkan functionality, be sure to touch all places mentioned
 above, only in inverse --- but usually @ref developers-deprecation "deprecate first",