docs: fix toc position scroll bug (#18868)

[michaelzusev]

SUMMARY

TOC position:sticky preventing users from scrolling vertically. This change makes TOC position:fixed in the custom.css.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Position:fixed for class theme name: '.theme-doc-toc-desktop' runs in both dev and build.

ADDITIONAL INFORMATION

• Has associated issue: Element on docs page preventing page to scroll down. #18868
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[villebro]

Thanks for fixing this! LGTM and tested to fix the issue.