Render Markdown in variable descriptions on new pipeline page

What does this MR do and why?

Render Markdown in Pipeline Variable descriptions, addressing part of the request in #441474 (closed)

References

• #441474 (closed)

MR acceptance checklist

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Add this CI configuration in your project:

   variables:
     SOME_VAR_NO_DESC:
       value: "test1, I am a variable that is not shown because I have no description"
     SOME_VAR_WITH_DESC:
       value: test2
       description: This is a variable
     SOME_VAR_WITH_MARKDOWN_DESC:
       value: test3
       description: This **is a variable**
     SOME_VAR_WITH_MARKDOWN_DESC_2:
       value: test4
       description: This is a [variable](https://docs.gitlab.com/ci/variables/)
     SOME_VAR_WITH_MARKDOWN_DESC_3:
       value: test5
       description: This is a `variable`, see [**here**](https://docs.gitlab.com/ci/variables/)
   
   job:
     script: echo test

2. Head to new pipeline page (/your/project/-/pipelines/new)
3. Observe Markdown descriptions rendered