📝 Add docs page for self-referential model

[daniil-berg]

Purpose

Adds a new page explaining how to construct a self-referential/recursive relationship with SQLModel to the Advanced User Guide section of the documentation. This is arguably a fairly common use case that has been asked about and discussed in issue #127 already.

Code

The example code uses a Villain class/model for demonstration in an attempt at honoring the Hero theme of the documentation. A villain can have a boss villain and villain minions.

Additional info

The new page also includes a brief explanation of the sa_relationship_kwargs parameter of the Relationship constructor, since that is essential for defining the desired relationship. Depending on how @tiangolo wants to structure the rest of the advanced guide, this section may either need to be expanded or instead abbreviated to a short mention and link to a dedicated section for sa_-style parameters in SQLModel fields and relationships in the future.

Update (2022-08-02)

Rebased onto most recent version of the main branch (0.0.8 release, 75ce455).

Then rebased again, after adding tests to cover the new documentation example code. Coverage of the added code should now be at 100% (see latest codecov checks).

[github-actions]

📝 Docs preview for commit e3dd424 at: https://6308770b7b8d7658d78c68dc--sqlmodel.netlify.app

[github-actions]

📝 Docs preview for commit 1860515 at: https://630879743f533d5c007b1ebc--sqlmodel.netlify.app

[codecov]

Codecov Report

Merging #408 (51d7d80) into main (75ce455) will decrease coverage by 0.77%.
The diff coverage is 0.00%.

❗ Current head 51d7d80 differs from pull request most recent head 5b4c9e4. Consider uploading reports for the commit 5b4c9e4 to get more accurate results

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
- Coverage   97.76%   96.99%   -0.78%     
==========================================
  Files         187      188       +1     
  Lines        6268     6318      +50     
==========================================
  Hits         6128     6128              
- Misses        140      190      +50     

Impacted Files	Coverage Δ
docs_src/advanced/self_referential/tutorial001.py	0.00% <0.00%> (ø)

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[github-actions]

📝 Docs preview for commit 51d7d80 at: https://6311b4a3b6b5535c3dfe9f74--sqlmodel.netlify.app

[github-actions]

📝 Docs preview for commit 6e10a41 at: https://6311b96eef8da65cd180f3e6--sqlmodel.netlify.app

[github-actions]

📝 Docs preview for commit 5b4c9e4 at: https://6311ba9503310f5f48cdd4f0--sqlmodel.netlify.app

[github-actions]

📝 Docs preview for commit c702e78 at: https://639ce0eaa12b8e030c9de393--sqlmodel.netlify.app

[jmehnle]

@tiangolo, what's blocking this from being merged?