[OUDS] Docs(utilities): add 'Flex' page

[julien-deramond]

Related issues

Listed in #2589.

Description

This PR adds the Utilities > Flex page based on:

• the previous corresponding Boosted page (see code source)
• the corresponding Bootstrap page (see code source)

To help the review, please note that the content of this page is exactly the same as the one from Boosted, and that the diff between this file and Bootstrap's one is:

--- a/site/content/docs/0.0/utilities/flex.md
+++ b/site/content/docs/0.0/utilities/flex.md
@@ -3,8 +3,6 @@ layout: docs
 title: Flex
 description: Quickly manage the layout, alignment, and sizing of grid columns, navigation, components, and more with a full suite of responsive flexbox utilities. For more complex implementations, custom CSS may be necessary.
 group: utilities
-aliases:
-  - "/docs/utilities/flex/"
 toc: true
 ---
 
@@ -636,7 +634,7 @@ Looking to replicate the [media object component](https://getbootstrap.com/docs/
 {{< example >}}
 <div class="d-flex">
   <div class="flex-shrink-0">
-    {{< placeholder width="100" height="100" color="#999" background="#e5e5e5" text="icon" >}}
+    {{< placeholder width="100" height="100" color="#999" background="#e5e5e5" text="Image" >}}
   </div>
   <div class="flex-grow-1 ms-3">
     This is some content from a media component. You can replace this with any content and adjust it as needed.
@@ -649,7 +647,7 @@ And say you want to vertically center the content next to the image:
 {{< example >}}
 <div class="d-flex align-items-center">
   <div class="flex-shrink-0">
-    {{< placeholder width="100" height="100" color="#999" background="#e5e5e5" text="icon" >}}
+    {{< placeholder width="100" height="100" color="#999" background="#e5e5e5" text="Image" >}}
   </div>
   <div class="flex-grow-1 ms-3">
     This is some content from a media component. You can replace this with any content and adjust it as needed.

Types of change

• New documentation (non-breaking change which adds functionality)

Live previews

• https://deploy-preview-2660--boosted.netlify.app/docs/0.0/utilities/flex

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[julien-deramond]

We might need to refine the colors used in this page.

[louismaximepiton]

LGTM, can be merged as-is.