[Code scan] Insert ICP footer into themes without a footer element

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

Problem

The extension advertises that it adds the ICP number to the footer, but the generic injection path only searches for a literal `` tag. Themes such as Alabaster render the footer as a `

` instead, so no ICP number is inserted.

Code references:

https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/README.md#L8-L13
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/inject.py#L85-L113
https://github.com/deepmodeling/deepmodeling_sphinx/blob/156679fe55fc61e8ec3b1e9be8a45fde3567b9cb/deepmodeling_sphinx/inject.py#L194-L205

Reproduction

Build a minimal Sphinx project with `html_theme = "alabaster"` and `extensions = ["deepmodeling_sphinx"]`. The generated HTML has `