doc: add northbound api arch docs

[qlyoung]

Adds the Northbound API docs from here.

This is a simple pandoc conversion of the docs, makefile update, and an small edit to make sure they show up in the toctree. I plan to come back and clean these up more, but this patch is years overdue.

Todo:

• Fix the heading levels to be correct
• Download all the images from postimg, put them in tree and fix the image references
• Remove the manual TOC blocks
• Probably other fixes

[NetDEF-CI]

Continuous Integration Result: FAILED

See below for issues.
CI System Testrun URL: https://ci1.netdef.org/browse/FRR-PULLREQ2-14241/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

Get source / Pull Request: Successful

Building Stage: Successful

Basic Tests: Failed

Topotests Ubuntu 18.04 arm8 part 8: Failed (click for details)

Topotests Ubuntu 18.04 arm8 part 8: Unknown Log URL: https://ci1.netdef.org/browse/FRR-PULLREQ2-14241/artifact/TOPO8U18AMD64/TopotestDetails/ Topotests Ubuntu 18.04 arm8 part 8: No useful log found

Successful on other platforms/tests

• Addresssanitizer topotests part 8
• Topotests debian 10 amd64 part 2
• Topotests Ubuntu 18.04 amd64 part 9
• Topotests Ubuntu 18.04 i386 part 9
• Static analyzer (clang)
• Addresssanitizer topotests part 6
• Topotests Ubuntu 18.04 amd64 part 1
• Topotests Ubuntu 18.04 i386 part 5
• Topotests Ubuntu 18.04 i386 part 0
• Ubuntu 18.04 deb pkg check
• Ubuntu 20.04 deb pkg check
• Topotests debian 10 amd64 part 1
• Topotests Ubuntu 18.04 amd64 part 8
• Topotests debian 10 amd64 part 7
• Topotests Ubuntu 18.04 arm8 part 0
• Topotests Ubuntu 18.04 arm8 part 5
• Topotests Ubuntu 18.04 arm8 part 6
• Addresssanitizer topotests part 0
• Topotests Ubuntu 18.04 arm8 part 1
• Topotests debian 10 amd64 part 6
• Topotests debian 10 amd64 part 3
• Topotests Ubuntu 18.04 amd64 part 6
• Debian 9 deb pkg check
• Topotests debian 10 amd64 part 5
• Addresssanitizer topotests part 1
• Topotests Ubuntu 18.04 arm8 part 3
• Topotests Ubuntu 18.04 i386 part 4
• Topotests debian 10 amd64 part 4
• Addresssanitizer topotests part 4
• Topotests debian 10 amd64 part 9
• Addresssanitizer topotests part 7
• Topotests Ubuntu 18.04 amd64 part 3
• Topotests Ubuntu 18.04 i386 part 7
• Topotests Ubuntu 18.04 i386 part 2
• Addresssanitizer topotests part 5
• Topotests Ubuntu 18.04 amd64 part 2
• Topotests Ubuntu 18.04 i386 part 1
• Topotests Ubuntu 18.04 i386 part 6
• Topotests Ubuntu 18.04 i386 part 8
• Topotests Ubuntu 18.04 amd64 part 0
• Topotests debian 10 amd64 part 8
• Topotests Ubuntu 18.04 arm8 part 4
• Debian 10 deb pkg check
• Addresssanitizer topotests part 2
• Topotests Ubuntu 18.04 arm8 part 9
• Topotests Ubuntu 18.04 amd64 part 7
• Topotests Ubuntu 18.04 amd64 part 5
• Topotests debian 10 amd64 part 0
• Addresssanitizer topotests part 3
• Topotests Ubuntu 18.04 arm8 part 2
• Topotests Ubuntu 18.04 arm8 part 7
• Topotests Ubuntu 18.04 i386 part 3
• CentOS 7 rpm pkg check
• Topotests Ubuntu 18.04 amd64 part 4
• Addresssanitizer topotests part 9

[mjstapp]

Great to have this brought in, thanks