📚 Multiple docs fixes and improvements

[daquinteroflex]

Live at https://docs.flexcompute.com/projects/tidy3d/en/demo-test-fix_docs_builds/development/documentation.html

• Fixes https://flow360.atlassian.net/browse/SCEM-7774
• Faster docs build
• Reduces number of sphinx warnings and docs issues
  • More consistent title levels
  • Adding missing documentation onto stub files
• Adds the latest lectures on the webcenter
• Ugly attr directory inclusion on each class in the pre/2.7 docs

Supersedes #1745

[daquinteroflex]

Hi @tylerflex, just fyi you've already reviewed/approved most of this in #1745 . This PR does the requested Jira fix and also removes the stuff that didn't work from the other old PR of the multiple docs improvements.

[tylerflex]

Thanks! it looks good to me. I'm not sure if you need someone else on the team to take a look? as I couldn't understand everything here. but as long as it fixes the issues I think it's good to me!

[daquinteroflex]

Sounds good I've added @jwang1231 who requested the Jira fix.

[jwang1231]

Double checked. The duplicated 'Documentation' has been removed from the tree node.

[daquinteroflex]

Nice, merging now and so this will be live on the next docs release.