[repo hygeine] Move everything mkdocs related to a dev dependency

[cmungall]

Continuing from

• upgrade mkdocs-mermaid2-plugin major version #353

In fact the version of mkdocs or any mkdocs plugin should be of zero concern to any consumer of sssom

The plugin is erroneously in the main dependencies:

sssom/pyproject.toml

Lines 12 to 19 in 2677726

	[tool.poetry.dependencies]
	python = "^3.8"
	linkml-runtime = "*"
	mkdocs-mermaid2-plugin = "^1.1.1"
	[tool.poetry.dev-dependencies]
	linkml = "^1.6.2"
	mkdocs-material = "^8.2.8"

[matentzn]

@hrshdhgd can you do this in a calm minute?

[hrshdhgd]

Great catch Nico. I should have waited to approve the PR... sorry. Fixing it right now.