docs: restored page on roles in security, executed other touchups, and…

[srinify]

… renamed docs readme.txt to readme.md

SUMMARY

Main change was restoring & tidying up the Roles / Security page.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

Fire up docs:

• cd docs/
• npm install
• npm run start

• navigate to localhost:8000`

• Test /docs/security page

ADDITIONAL INFORMATION

No change to Superset (just docs!)

[codecov-io]

Codecov Report

Merging #11978 (6340f8a) into master (9256b6f) will decrease coverage by 3.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11978      +/-   ##
==========================================
- Coverage   63.59%   60.37%   -3.23%     
==========================================
  Files         940      906      -34     
  Lines       45575    44641     -934     
  Branches     4373     4038     -335     
==========================================
- Hits        28984    26951    -2033     
- Misses      16415    17690    +1275     
+ Partials      176        0     -176     

Flag	Coverage Δ
cypress	53.20% <ø> (?)
javascript	?
python	64.23% <ø> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/security/manager.py	90.41% <ø> (-0.60%)	⬇️
...uperset-frontend/src/dashboard/util/dnd-reorder.js	0.00% <0.00%> (-100.00%)	⬇️
...rset-frontend/src/dashboard/util/getEmptyLayout.js	0.00% <0.00%> (-100.00%)	⬇️
...dashboard/components/resizable/ResizableHandle.jsx	0.00% <0.00%> (-100.00%)	⬇️
.../src/dashboard/util/getFilterScopeFromNodesTree.js	0.00% <0.00%> (-93.48%)	⬇️
...src/dashboard/components/gridComponents/Header.jsx	10.52% <0.00%> (-86.85%)	⬇️
...rc/dashboard/components/gridComponents/Divider.jsx	13.33% <0.00%> (-86.67%)	⬇️
...ontend/src/dashboard/util/getDashboardFilterKey.ts	14.28% <0.00%> (-85.72%)	⬇️
...end/src/SqlLab/components/ExploreResultsButton.jsx	8.00% <0.00%> (-84.00%)	⬇️
...nd/src/views/CRUD/data/query/QueryPreviewModal.tsx	14.70% <0.00%> (-82.97%)	⬇️
... and 442 more

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