improvement: if social card not found, try to retrieve length from remote URL

[Guts]

If the social card has been previously published, it works.

[codecov]

Codecov Report

Merging #225 (132f9a9) into main (e505ce2) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #225   +/-   ##
=======================================
  Coverage   83.96%   83.96%           
=======================================
  Files          10       10           
  Lines         524      524           
  Branches      108      108           
=======================================
  Hits          440      440           
  Misses         53       53           
  Partials       31       31           

Files	Coverage Δ
mkdocs_rss_plugin/util.py	72.76% <100.00%> (ø)