Fix links

[rvsia]

• replace obsolete github links
• make all internal links as NavLink
• some little typos fixing and make header smaller (first and second heading levels are too big, it doesn't look good)

[codecov-io]

Codecov Report

Merging #141 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #141   +/-   ##
=======================================
  Coverage   83.24%   83.24%           
=======================================
  Files          83       83           
  Lines        1355     1355           
  Branches      319      319           
=======================================
  Hits         1128     1128           
  Misses        195      195           
  Partials       32       32

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