Serve index.html and index.php by default for ViMbAdmin

Summary:
This is a follow-up for ecc331a6.

Ref T1932.

Test Plan:
This commits a hotfix in production from 2024-07-04 (according stat)
on Hervil.

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: DorianWinty

Maniphest Tasks: T1932

Differential Revision: https://devcentral.nasqueron.org/D3556