Link development and design documentation

[mkindahl]

The tree contains a lot of design and architecture documents, but they
are not linked together, so this commits adds a few additional README
and build a basic structure for the documentation.

[codecov]

Codecov Report

Merging #5050 (d6905ea) into main (38878be) will decrease coverage by 1.33%.
The diff coverage is 89.55%.

❗ Current head d6905ea differs from pull request most recent head f90d6b6. Consider uploading reports for the commit f90d6b6 to get more accurate results

@@            Coverage Diff             @@
##             main    #5050      +/-   ##
==========================================
- Coverage   90.92%   89.59%   -1.34%     
==========================================
  Files         224      227       +3     
  Lines       42788    51607    +8819     
==========================================
+ Hits        38905    46235    +7330     
- Misses       3883     5372    +1489     

Impacted Files	Coverage Δ
src/chunk.h	100.00% <ø> (ø)
src/chunk_index.h	100.00% <ø> (ø)
src/compat/compat.h	49.20% <0.00%> (-43.78%)	⬇️
src/dimension_vector.c	87.50% <0.00%> (+1.13%)	⬆️
src/guc.c	94.00% <0.00%> (-6.00%)	⬇️
src/nodes/hypertable_modify.c	35.78% <0.00%> (-34.51%)	⬇️
src/planner/planner.h	100.00% <ø> (ø)
src/ts_catalog/catalog.h	100.00% <ø> (ø)
src/ts_catalog/continuous_agg.h	100.00% <ø> (ø)
src/utils.h	80.00% <ø> (-2.61%)	⬇️
... and 301 more

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