Fix links to 'Reaching The Ground With Lightning'

[ekzyis]

Hello!

I noticed that the links to 'Reaching The Ground With Lightning' in README.md and CONTRIBUTING.md were broken because the file was moved in ElementsProject/lightning@9e47c16.

I now used a permalink so the links will not break again. However, that also means if the PDF gets updated, the links will still point to the old version.

However, since the PDF has not been changed meaningfully in 10 years (last time in ElementsProject/lightning@2ab9e3b), I don't think that's ever going to happen again.

But if it happens, I think working links are more important anyway.

I also searched for deployable-lightning.pdf to check if there's another broken link, but I didn't find any.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.94%. Comparing base (f93d02f) to head (8ac4757).
⚠️ Report is 57 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3219      +/-   ##
==========================================
+ Coverage   86.43%   88.94%   +2.50%     
==========================================
  Files         242      218      -24     
  Lines       22607    20500    -2107     
  Branches      832      800      -32     
==========================================
- Hits        19541    18233    -1308     
+ Misses       3066     2267     -799     

see 17 files with indirect coverage changes

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