Add historical TLE files link

[zxdawn]

Add historical TLE files link to the doc.

• Closes #xxxx
• Tests added
• Tests passed
• Passes flake8 pyorbital
• Fully documented

[codecov]

Codecov Report

Merging #82 (c88b79d) into main (91794a6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #82   +/-   ##
=======================================
  Coverage   85.31%   85.31%           
=======================================
  Files          13       13           
  Lines        1893     1893           
=======================================
  Hits         1615     1615           
  Misses        278      278           

Flag	Coverage Δ
unittests	85.31% <ø> (ø)

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[djhoese]

I don't see any issues with this. Thanks! @pnuu or @mraspaud feel free to complain after the fact.