docs: documentation for version 6

The goal is to have almost finished documentation for version 6.

Current documentation can be seen with gitlab pages. (generated on-demand from branches chosen by us)

Step 1: Writing the documentation

The structure of documentation is based on #776 (closed). Some related comments can be found in !1377 (merged).

• Getting Started section: installation, startup, initial configuration (examples)
• Configuration section: rewrite pages about Lua configuration with declarative configuration
  • syntax and conventions (this might be already rewritten somewhere)
  • modules
  • networking
  • performance and resiliency
  • policy, access control and data manipulation
  • logging, monitoring, diagnostics
  • DNSSEC, data verification
  • experimental features
• Management section
  • HTTP API
  • kresctl utility
• For operators section
  • upgrading to version 6
• For developers section
  • internal architecture
  • rework doc/dev/build.rst to describe one supported way of building Knot Resolver
• Deployment guides
  • manual
  • systemd
  • docker
  • multiple instances
• extending the resolver
• create gitlab issues for all documentation sections that won't be fully completed with this MR

Step 2: Collect and implement feedback

1. run spell checker
2. collect feedback from @vcunat
3. implement feedback
4. collect feedback from @llhotka
5. implement feedback
6. collect feedback from someone unrelated to the dev team (ODVR admins, someone random, ...)
7. implement feedback

Related !1377 (merged) Closes #776 (closed)