Add new section: Naming Conventions

[kfogel]

Right now, this new section just holds one subsection, documenting one convention: "id vs _id: ID as Primary Key vs Foreign Key" (as discussed in PR #345). There are probably other naming conventions in the Arches code, though, and now at least there's a place to document them when we think of them (although it remains to be seen whether "Data Model" is the right place for all this -- perhaps the entire "Naming Conventions" section will need to be moved eventually).

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• [x] the PR branch was originally made from the base branch
• [x] after these changes the docs build locally without error

[kfogel]

@mradamcox, @ekansa: ready for review, as promised in PR #345. I looked around but I'm still not sure I've put this new content in the best place; if there's a better location for it in the docs, feel free to just point me to it and I'll update this PR accordingly.

[kfogel]

Thanks for the fast review, @ekansa!

[kfogel]

W00t! Thank you, @ekansa.