[Index management] Document option to convert to lookup index from the "Manage index" menu

[florent-leborgne]

Version: 9.2
PR with details: elastic/kibana#233998

[florent-leborgne]

CC @elastic/admin-docs, just scanned some of the Kibana repo's PRs for 9.2 to catch things that may not have made their way to us. Feel free to close if that duplicates something already on your table.

[shainaraskas]

relevant? #3317

[kilfoyle]

I can take this one for the upcoming sprint. In addition to the new "convert to lookup" option, there likely are other things on that "Manage index" menu that we don't cover on the Manage indices page, so the page could use some love.