docs: added documentation folder for extensions

[dackers86]

Storage/implementation of official extensions documentation

fixes: https://github.com/firebase/extensions/issues/949 fixes: https://github.com/firebase/extensions/issues/963

[codecov-commenter]

Codecov Report

Merging #961 (2936cde) into next (02e7fea) will decrease coverage by 0.23%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             next     #961      +/-   ##
==========================================
- Coverage   47.16%   46.94%   -0.22%     
==========================================
  Files          36       36              
  Lines        1514     1632     +118     
  Branches      225      261      +36     
==========================================
+ Hits          714      766      +52     
- Misses        799      865      +66     
  Partials        1        1              

Impacted Files	Coverage Δ
storage-resize-images/functions/src/index.ts	27.09% <0.00%> (-0.38%)	:arrow_down:
...torage-resize-images/functions/src/resize-image.ts	43.08% <0.00%> (+2.38%)	:arrow_up:
firestore-send-email/functions/src/logs.ts	63.34% <0.00%> (+9.49%)	:arrow_up:

[Salakar]

Lets rename .docs to docs also

[i14h]

@kevinthecheung can you please take a look at these docs? we intend to merge them with firebase documentation as well

[dackers86]

Thanks @kevinthecheung. Suggestions committed.