add LangSmith API docs to mintlify

[Lauren Hirata Singh (lnhsingh)]

Fixes DOC-878

[github-actions]

Thanks for opening a docs PR, Lauren Hirata Singh (@lnhsingh)! When it's ready for review, please add the relevant reviewers:

• @katmayb or @fjmorris (LangSmith)

[github-actions]

❌ docs.json validation failed

This PR has issues with docs.json: either pages were removed without redirects, or some pages reference non-existent source files.

 The following pages were removed from docs.json without adding redirects:
  - langsmith/smith-api-ref

Please add a redirect for each removed page to the `redirects` array in docs.json.
Example: {"source": "/path/to/removed-page", "destination": "/path/to/new-location"}

Edit (Lauren): This can be ignored bc the removed page was a redirecting page

[github-actions]

Mintlify preview ID generated: preview-lsapi-1776271929-1f625bc

[github-actions]

Mintlify preview ID generated: preview-lsapi-1776436970-ca6ef08