doc: add info on syncing to origin/main and rebasing

[terriko]

Adding some information for common problems first time contributors have:

• keeping their main branch in sync with origin/main . Previously we said it was a good idea but didn't include info on what to do if you accidentally commit to that branch)
• rebasing to origin/main. Just saying that it comes up sometimes in code review and giving a link to a more extensive document on what git rebase means.

[codecov-commenter]

Codecov Report

Merging #1540 (481c099) into main (b2262e0) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1540      +/-   ##
==========================================
+ Coverage   79.18%   79.27%   +0.09%     
==========================================
  Files         281      281              
  Lines        5548     5548              
  Branches      905      905              
==========================================
+ Hits         4393     4398       +5     
+ Misses        971      966       -5     
  Partials      184      184              

Flag	Coverage Δ
longtests	79.27% <ø> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
cve_bin_tool/nvd_api.py	87.75% <0.00%> (+5.10%)	⬆️

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[terriko]

I'm going to take @BreadGenie 's thumbs up above as sufficient documentation review and go ahead and merge this. Thanks!