docs: guide for existing dbt projects

[pnadolny13]

Closes #6704

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[codecov]

Codecov Report

Merging #6769 (393477d) into main (2052ede) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6769      +/-   ##
==========================================
+ Coverage   88.56%   88.58%   +0.02%     
==========================================
  Files         280      280              
  Lines       20260    20260              
  Branches     2002     2002              
==========================================
+ Hits        17943    17948       +5     
+ Misses       1978     1975       -3     
+ Partials      339      337       -2     

Impacted Files	Coverage Δ
src/meltano/core/elt_context.py	95.89% <0.00%> (+0.68%)	⬆️
src/meltano/cli/elt.py	95.02% <0.00%> (+1.10%)	⬆️
src/meltano/cli/state.py	89.93% <0.00%> (+1.34%)	⬆️

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