use relative and absolute URLs

[MuxaJIbI4]

see issue #830 for full description.

Now test website (with not empty path after host) looks and works correctly (CSS) => https://muxajibi4.github.io/git.github.io/

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Not fixed links in older files:

1. ignored (for now?) absolute links (with host) to https://git.github.io in files:

• all .markdown files /_posts
• multiple .md files in /links

2. Didn't add relative_url in links to '/images/*' in files:

• Git-Logo-1788C.png
  SoC-2016-Org-Application.md
  SoC-2017-Org-Application.md
  SoC-2018-Org-Application.md
  SoC-2019-Org-Application.md
  SoC-2020-Org-Application.md
  SoC-2021-Org-Application.md
  SoC-2022-Org-Application.md

• git_merge_2020_cake.jpg
  2020-03-25-edition-61.markdown

• git_merge_2020_timeline.jpg
  2020-03-25-edition-61.markdown

• logo.png
  SoC-2014-Org-Application.md
  SoC-2015-Org-Application.md