docs: replace relative URLs in PyPI documentation

[njzjz]

Currently, all images and relative links in PyPI documentation documentation are broken, as PyPI doesn't support relative URLs:

This PR replaces them with GitHub absolute URLs in the PKG-INFO file of the generated source distribution and wheels.

[<picture><source media="(prefers-color-scheme: dark)" srcset="https://github.com/deepmodeling/deepmd-kit/raw/master/./doc/_static/logo-dark.svg"><source media="(prefers-color-scheme: light)" srcset="https://github.com/deepmodeling/deepmd-kit/raw/master/./doc/_static/logo.svg"><img alt="DeePMD-kit logo" src="https://github.com/deepmodeling/deepmd-kit/raw/master/./doc/_static/logo.svg"></picture>](https://github.com/deepmodeling/deepmd-kit/tree/master/./doc/logo.md)

- [About DeePMD-kit](https://github.com/deepmodeling/deepmd-kit/tree/master/#about-deepmd-kit)

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (cf61140) 75.46% compared to head (e6a36ec) 75.46%.

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #2885   +/-   ##
=======================================
  Coverage   75.46%   75.46%           
=======================================
  Files         244      244           
  Lines       24518    24518           
  Branches     1580     1580           
=======================================
  Hits        18503    18503           
  Misses       5084     5084           
  Partials      931      931           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.