Minor: Add Development Environment to Documentation Index

[alamb]

Which issue does this PR close?

Rationale for this change

I found the that the setting up development environment page wasn't linked in the table of contents

running cd docs && ./build.sh also shows an error

reading sources... [100%] user-guide/sql/write_options
/Users/andrewlamb/Software/datafusion/docs/temp/index.rst:138: WARNING: toctree contains reference to nonexisting document 'contributor-guide/getting_started'
looking for now-outdated files... none found
pickling environment... done
checking consistency... /Users/andrewlamb/Software/datafusion/docs/temp/contributor-guide/development_environment.md: WARNING: document isn't included in any toctree

I think this may have been introduced in #14694

What changes are included in this PR?

Fix the index:

Are these changes tested?

I tested them lcal

Are there any user-facing changes?

[dentiny]

Oh I should be more careful, thank you!

[alamb]

Oh I should be more careful, thank you!

No worries -- it is easy to fix. Maybe we should turn doc build warnings into errors (like we do for code 🤔 )

[comphead]

lgtm thanks @alamb for finding this