feat(docs/en/volunteers.mdx): translate from Chinese version

Signed-off-by: Haonan Chen <chn@chn.moe>

src/content/docs/en/volunteers.mdx

@@ -13,7 +13,7 @@ We are recruiting volunteers for Nix CN Meetup #2!
     Fill out the Volunteer Application Form (Chinese)
 </LinkButton>
 
-Open positions are as follows:
+Open roles are as follows:
 
 - On-site Support (3 people)
     - Handle check-in and souvenir distribution
@@ -26,5 +26,5 @@ Open positions are as follows:
     - Heavy lifting involved.
 - Venue Setup (2 people)
     - Please arrive at the venue 3 days before the event starts.
-- Floater (≤10 people)
+- Flexible (≤10 people)
     - Tasks assigned based on real-time needs.