[web] Preparations for new beginners guide

• Docs overview page has now some categories: https://ogs.ogs.xyz/-/ogs/-/jobs/274545/artifacts/web/public/docs.html
• TOC on the left-hand side can now be set to be collapsed: https://ogs.ogs.xyz/-/ogs/-/jobs/274545/artifacts/web/public/docs/devguide/getting-started/introduction.html
• Moved process-dependent docs to its own section: https://ogs.ogs.xyz/-/ogs/-/jobs/274545/artifacts/web/public/docs/processes/heat-transport/heat_transport_bhe.html. This in preparation of @felikskiszkurno new beginner guide.