[Code scan] Render Docs dropdown subitems in the mobile menu

This issue is a result of a Codex global code scan of deepmodeling/deepmodeling_sphinx at commit 156679fe55fc61e8ec3b1e9be8a45fde3567b9cb.

Problem

The desktop banner renders `item.subitem` entries under the Docs dropdown, but the mobile menu only renders the top-level items. On mobile, users cannot directly reach DeePMD-kit, DP-GEN, dpdata, DPDispatcher, ABACUS, DeepFlame, or DPTI from the banner menu.

Code references:

https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/config.py#L5-L26
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/banner.html#L20-L53
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/banner.html#L83-L92

Impact

The mobile navigation is functionally less complete than desktop navigation and hides the project links that are most useful from the Docs section.

Suggested fix

Render submenu items in the mobile menu as well, either as an expanded nested list under Docs or as an accessible collapsible section matching the desktop dropdown contents.

Start by comparing the desktop and mobile menu sections in deepmodeling_sphinx/banner.html, using deepmodeling_sphinx/config.py to identify the Docs subitems. Update the mobile navigation so it exposes the same listed project links in an accessible nested or collapsible section, then verify the links are reachable in a mobile-sized view.

Written by the indexing model from the issue text.