Docs: add mermaid diagram support

[rkingsbury]

This PR adds support for mermaid diagrams to maggma and adds a small diagram to concepts.md in the docs that illustrates the relationship between Store and Builder. I envision mermaid diagrams being very useful for future docs updates as a way of illustrating the steps in a build pipeline.

This introduces no new dependencies; it simply requires a small configuration change in mkdocs.yml

[codecov]

Codecov Report

Merging #677 (02df72f) into main (b506eb8) will not change coverage.
The diff coverage is n/a.

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  Hits         2726     2726           
  Misses        322      322           

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[rkingsbury]

@munrojm friendly reminder about this; what do you think? I'm about to introduce maggma to some colleagues and I think having a diagram in the docs that illustrates how Store and Builder work together would be really helpful to newcomers.

[munrojm]

@rkingsbury, sorry I didn't circle back on this. I think this is a great addition. Did you want to add more figures in this PR or in a new one?

[rkingsbury]

Thanks @munrojm ! I think this is a good start as-is. Once support for mermaid in the docs is turned on we can easily add more as other parts of the docs evolve

[munrojm]

Sounds good!