DOC-3247 Fix documentation in main branch: increase toctree maxdepth to 3

[sfc-gh-mbartolome]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes DOC-3247 which is related to SNOW-1453518

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.

3. Please describe how your code solves the related issue.

   This same fix was made in SNOW-1453518 Fix documentation for v1.17.0 - increase toctree maxdepth to 3 #1708 and SNOW-1453518 Fix documentation in v1.18.0 #1711 for the v1.17.0 and v1.18.0 branches, but not yet in main so the sidebar navigation issue was reintroduced again in the 1.19.0 release.

   Refer to thread: https://snowflake.slack.com/archives/C049VF99031/p1717087196127959?thread_ts=1716580744.074129&cid=C049VF99031

   • Increased the toctree maxdepth from 2 to 3 - this should make the sidebar navigation work as expected (drop downs).
   • Removed duplicate session documentation link for index.rst for Snowpark APIs - it should still exist in the landing page.

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[sfc-gh-mbartolome]

I have read the CLA Document and I hereby sign the CLA

[sfc-gh-mbartolome]

@sfc-gh-vbudati @sfc-gh-jrose thanks for the review!

Looks like I don't have access to merge into main in this repo – would one of you mind merging it for me?