feat: add gatsby-remark-autolink-headers support

[tonyz0x0]

Description

The PR is aimed to add gatsby-remark-autolink-headers plugin to support markdown TOC.

[codecov]

Codecov Report

Merging #118 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #118   +/-   ##
======================================
  Coverage    95.1%   95.1%           
======================================
  Files          28      28           
  Lines         143     143           
  Branches        8       8           
======================================
  Hits          136     136           
  Misses          7       7

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[alxshelepenok]

You can also add CSS in _generic.scss to align the headers:

.anchor {
  margin-left: -30px !important;
  padding-right: 14px !important;
}

Demo

[tonyz0x0]

@alxshelepenok Thanks a lot.