Improve user and meta documentation

.ci/README.md

.gitlab/merge_request_templates/MR_from_task.md

CHANGELOG.md

CHANGELOG.md

CODE_OF_CONDUCT.md

CONTRIBUTING.md

README.md

doc/docusaurus/README.md

doc/docusaurus/docs/CHANGELOG.md

doc/docusaurus/docs/CODE_OF_CONDUCT.md

doc/docusaurus/docs/CONTRIBUTING.md

doc/docusaurus/docs/about.md

doc/docusaurus/docs/api.md

doc/docusaurus/docs/community.md

doc/docusaurus/docs/config_intro.md

doc/docusaurus/docs/ini_file.md

doc/docusaurus/docs/input_data.md

doc/docusaurus/docs/install_use.md

doc/docusaurus/docs/math_expr.md

doc/docusaurus/docs/math_model.md