[docs] Move dbnode docs to centralized location.

[notbdu]

What this PR does / why we need it:

Moves dbnode docs to centralized location.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov[bot]]

Codecov Report

Merging #3111 (e8b65c2) into master (9be8ef9) will decrease coverage by 0.1%. Report is 823 commits behind head on master. The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #3111     +/-   ##
=========================================
- Coverage    72.2%    72.2%   -0.1%     
=========================================
  Files        1084     1084             
  Lines      100290   100188    -102     
=========================================
- Hits        72486    72399     -87     
+ Misses      22772    22745     -27     
- Partials     5032     5044     +12     

Flag	Coverage Δ
aggregator	75.8% <ø> (+<0.1%)	:arrow_up:
cluster	84.8% <ø> (+<0.1%)	:arrow_up:
collector	84.3% <ø> (ø)
dbnode	78.7% <ø> (+<0.1%)	:arrow_up:
m3em	74.4% <ø> (ø)
m3ninx	73.2% <ø> (+<0.1%)	:arrow_up:
metrics	20.0% <ø> (ø)
msg	74.0% <ø> (-0.2%)	:arrow_down:
query	67.2% <ø> (ø)
x	80.2% <ø> (-0.3%)	:arrow_down:

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[ChrisChinchilla]

Thanks for this @notbdu I actually have some other elements of this PR in progress elsewhere, so I am going to park it here for now, but will get back to merging it a little later.