Add FastAPI official docs and free tutorial (Python resources) — responds to #12043

[akrutitiwari24]

Issue #12043 requested adding FastAPI learning resources. The requested O'Reilly title "FastAPI: Modern Python Web Development" appears to be a paid book, so instead this PR adds two free, authoritative FastAPI resources:

• FastAPI official docs: https://fastapi.tiangolo.com/ (tutorial + reference)
• Tutorialspoint FastAPI PDF: https://tutorialspoint.com/fastapi/fastapi_tutorial.pdf

Both are free and legal to link to; added them alphabetically to the Python section in books/free-programming-books-langs.md.

I verified both links on 2025-10-30. See #12043 for the original request.

[github-actions]

Linter failed, fix the error(s):

free-programming-books/books/free-programming-books-langs.md
2118:1-2140:109  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2      remark-lint
2144:1-2145:169  warning  Alphabetical ordering: swap l.2145 and l.2144                          alphabetize-lists      remark-lint
 2144:3-2144:70  warning  Remove trailing slash (https://fastapi.tiangolo.com)                   no-url-trailing-slash  remark-lint
2145:3-2145:169  warning  Missing PDF indication                                                 books-links            remark-lint

[github-actions]

Linter failed, fix the error(s):

free-programming-books/books/free-programming-books-langs.md
2118:1-2140:109  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2  remark-lint

[github-actions]

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