chore(docs): fix readthedocs build (#14253)

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## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
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- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: taegyunkim

.readthedocs.yml

@@ -5,6 +5,7 @@ build:
     python: "3.10"
     rust: "1.86"
   commands:
+    - cargo install --force --root /home/docs/.asdf --git https://github.com/DataDog/libdatadog --bin dedup_headers tools
     - git fetch --unshallow || true
     - curl -L https://github.com/pypa/hatch/releases/download/hatch-v1.12.0/hatch-x86_64-unknown-linux-gnu.tar.gz | tar zx
     - install -t $HOME/.asdf/bin hatch