installation guide: update URL to pipx installation docs

[kenyon]

The old URL doesn't redirect to the installation section.

[ansible-documentation-bot[bot]]

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[patchback[bot]]

Backport to stable-2.17: 💚 backport PR created

✅ Backport PR branch: patchback/backports/stable-2.17/ab7a4598fd6cda02537016296df9bb6cbd521789/pr-3248

Backported as https://github.com/ansible/ansible-documentation/pull/3316

🤖 @patchback I'm built with octomachinery and my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback[bot]]

Backport to stable-2.18: 💚 backport PR created

✅ Backport PR branch: patchback/backports/stable-2.18/ab7a4598fd6cda02537016296df9bb6cbd521789/pr-3248

Backported as https://github.com/ansible/ansible-documentation/pull/3317

🤖 @patchback I'm built with octomachinery and my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback[bot]]

Backport to stable-2.19: 💚 backport PR created

✅ Backport PR branch: patchback/backports/stable-2.19/ab7a4598fd6cda02537016296df9bb6cbd521789/pr-3248

Backported as https://github.com/ansible/ansible-documentation/pull/3318

🤖 @patchback I'm built with octomachinery and my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback[bot]]

Backport to stable-2.20: 💚 backport PR created

✅ Backport PR branch: patchback/backports/stable-2.20/ab7a4598fd6cda02537016296df9bb6cbd521789/pr-3248

Backported as https://github.com/ansible/ansible-documentation/pull/3319

🤖 @patchback I'm built with octomachinery and my source is open — https://github.com/sanitizers/patchback-github-app.