Fix #4550: html: Centering tables by default using CSS

[tk0miya]

Feature or Bugfix

• Feature

Purpose

• To make table alignment customizable, use align-default class to align tables in HTML.
• refs: Tables should be centered by default. #4550

[codecov]

Codecov Report

Merging #6341 into 2.0 will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              2.0    #6341      +/-   ##
==========================================
- Coverage   83.58%   83.55%   -0.03%     
==========================================
  Files         278      280       +2     
  Lines       39997    40031      +34     
  Branches     5916     5919       +3     
==========================================
+ Hits        33430    33447      +17     
- Misses       5232     5246      +14     
- Partials     1335     1338       +3

Impacted Files	Coverage Δ
tests/test_build_html.py	97.52% <ø> (ø)	⬆️
sphinx/writers/latex.py	81.65% <ø> (ø)	⬆️
tests/test_ext_graphviz.py	100% <100%> (ø)	⬆️
sphinx/transforms/__init__.py	88.38% <100%> (ø)	⬆️
tests/test_ext_inheritance_diagram.py	100% <100%> (ø)	⬆️
sphinx/__init__.py	73.91% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

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