[Code scan] Add accessible names to mobile menu controls

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

Problem

The mobile menu open and close controls are empty anchor elements that rely on background images. They do not have text content or `aria-label` attributes, so assistive technologies can expose them as unnamed links.

Code references:

https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/banner.html#L56-L60
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/banner.html#L74-L78
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/banner.css#L274-L286
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/banner.css#L459-L472

Impact

Screen-reader and voice-control users may not be able to identify or operate the menu controls reliably.

Suggested fix

Use `` elements for these controls, or add accessible names such as `aria-label="Open menu"` and `aria-label="Close menu"`. The open control should also expose menu state with `aria-expanded` and reference the controlled menu where practical.