Hugo docs: Add dedicated walk-throughs for VM.build and xenguest

[bernhardkaindl]

Add walk-throughs for VM_build (split into 3 pages for improved focus):

The existing walk-though for VM_build as been uses as the basis and extended: The parts which are currently in Chapter 3 of the VM.start workflow have been extracted, moved to new files and extended.

A special improvement is that the deep nesting of lists is split into separate chapters and flattened, which fixes the deep nesting that I initially used.

The new extracted and improved walk-though contains flowcharts that show the execution flow and a call graph to visualise the content of the walk-through.

This PR has 3 commits, the 2nd commit is the big commit.
The 1st and 3rd commit are very small and only minor improvements:

1. doc/hugo.toml: Use the theme font for mermaid diagrams too
   This is only a cosmetic change to align the font of the Mermaid diagrams
   with the font of the Hugo Relearn theme. It makes the look of the Mermaid
   diagrams more consistent with the more modern theme of the site.
   It is only a very minimal step and very small.
2. docs: Add dedicated walk-throughs for VM.build and xenguest
   Add walk-throughs for VM_build (split into 3 pages for improved focus).
3. xenopsd docs: Add Walk-through descriptions, show them on the index page
   Show a short on-line summary of the workflows on the index walk-throug index page

[bernhardkaindl]

You can preview the new pages of the PR (with the font and the Walk-through descriptions) here:
https://bernhard-xapi-onrender-com-pr-10.onrender.com/xenopsd/walkthroughs/index.html

This is an extract of the index from this link which shows the new pages:

• Starting a VM
  Complete walkthrough of starting a VM, from receiving the request to unpause.
• Building a VM
  After VM_create, VM_build builds the core of the domain (vCPUs, memory)
  • VM_build μ-op
    Overview of the VM_build μ-op (runs after the VM_create μ-op created the domain).
  • Domain.build
    Prepare the build of a VM: Wait for scrubbing, do NUMA placement, run xenguest.
  • xenguest
    Perform building VMs: Allocate and populate the domain's system memory.

[lindig]

This is great.

[psafont]

I really like the change!

Maybe mentioning all the chain of functions is not needed and will easily become out-of-date, but this is very positive nonetheless

[stormi]

This will make for a good read.

CC @gthvn1