docs: community detection guide

[BeaMarton13]

Copilot was used to assist with the generation of some functions and explanatory texts.

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.

[szhorvat]

It's starting to look great!

Some tips:

• Be sure to add necessary packages to the install and update steps of mkdocs.sh
• Can we find a way to add a download button to the end of pages, to allow people to get the notebook?
• Note that this is integrated into the doc pages, can we link directly to function docs? If this is easy, let's do it, if not, it's not worth spending time on.
• Check the visual output and use only as many bullet points as looks good. I don't suggest trying to include notebook cells in bullet points. Just write explanations above cells without a bullet.

[szhorvat]

Be sure to add necessary packages to the install and update steps of mkdocs.sh

@BeaMarton13, please be sure to do this, otherwise the docs won't build, and we can't easily check them.