manager: some parts of documentation for version 6

Unfinished parts from this MR are tracked in #796.

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Goal of this MR is to have a almost finished documentation for version 6. Parts that will not be finished should have a linked issue created by the time this is merged.

GitLab pages with documentation (probably, but not necessarily from this branch)

Step 1

• getting started 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
• for developers section
  • internal architecture
• deployment guide
  • 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

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