[DOC] Improve the HTML documentation

[tbouffard]

To do after #40

• [x] name of the package: currently bpmnVisualization --> put the final name!!!! for instance bpmnVisualizationR: #172
• [ ] no license link to the generated license page (check adding + file LICENSE in DESCRIPTION), see https://patchwork.data-imaginist.com/reference/index.html. See also https://r-pkgs.org/license.html#sec-more-licenses
• [ ] authors: link to GitHub accounts? https://pkgdown.r-lib.org/reference/build_home.html?q=title#yaml-config-authors
• [ ] configure favicon: https://pkgdown.r-lib.org/reference/build_favicons.html
• [ ] display badges on the right side of the home page: https://pkgdown.r-lib.org/reference/build_home.html#dev-badges. Example: https://patchwork.data-imaginist.com/reference/index.html
• [ ] customization: see https://pkgdown.r-lib.org/articles/customise.html but don't take too much time on theming for the first implementation
• [ ] code of conduct, we don't have one in the repository, so it won't be displayed on the sidebar of the home page, but we could display it at the bottom of the home page (link to the .github repository) like in cf https://patchwork.data-imaginist.com/reference/index.html. This is what is done in #147
• [ ] reference page: introduce sections (reorganize the functions list) like in https://patchwork.data-imaginist.com/reference/index.html. See https://pkgdown.r-lib.org/articles/pkgdown.html#reference
• [ ] news page: decide if we introduce a NEWS.md file, if we reference the release notes of the minor/major version in the news menu. See https://pkgdown.r-lib.org/reference/build_news.html. If we do this, update the release process to add the news entry in _pkgdown.yml
• [ ] Configure url (https://pkgdown.r-lib.org/articles/pkgdown.html#metadata): manage url for gh-pages, surge and local
• [ ] Document how to publicise the release: https://r-pkgs.org/release.html#promotion

Fix generation warnings

Reading 'MAINTAINERS.md' Writing 'MAINTAINERS.html' Warning: Missing images in '/mnt/toma/SharedData/development/process-analytics/repo/bpmn-visualization-R/MAINTAINERS.md': './doc/maintainers/release_cran_source_package.png' ℹ pkgdown can only use images in 'man/figures' and 'vignettes'