docs: fix styles and add algolia search classes

[pkdotson]

SUMMARY

1. add search classes for algolia serach
2. update default styles of menu component to superset brand color.
3. update media query styles to keep images and font inside container on mobile view.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

Visual manual qa testing of new css.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #12267 (f4f86df) into master (9eb911d) will decrease coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12267      +/-   ##
==========================================
- Coverage   67.14%   66.95%   -0.19%     
==========================================
  Files        1002     1002              
  Lines       49300    49300              
  Branches     5010     5010              
==========================================
- Hits        33104    33011      -93     
- Misses      16071    16164      +93     
  Partials      125      125              

Flag	Coverage Δ
cypress	51.44% <ø> (ø)
javascript	61.20% <ø> (ø)
python	63.84% <ø> (-0.32%)	⬇️

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

Impacted Files	Coverage Δ
superset/sql_validators/postgres.py	50.00% <0.00%> (-50.00%)	⬇️
superset/db_engine_specs/mysql.py	79.59% <0.00%> (-12.25%)	⬇️
superset/db_engine_specs/sqlite.py	65.62% <0.00%> (-9.38%)	⬇️
superset/databases/commands/create.py	83.67% <0.00%> (-8.17%)	⬇️
superset/databases/commands/update.py	85.71% <0.00%> (-8.17%)	⬇️
superset/utils/celery.py	96.42% <0.00%> (-3.58%)	⬇️
superset/models/core.py	85.86% <0.00%> (-2.99%)	⬇️
superset/views/core.py	72.89% <0.00%> (-2.48%)	⬇️
superset/datasets/api.py	89.44% <0.00%> (-1.84%)	⬇️
superset/db_engine_specs/base.py	84.27% <0.00%> (-1.81%)	⬇️
... and 8 more

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