I understand an index is different from searching (an index lists terms you may not otherwise think of to search) - but addressing your concern about searching -- use a search engine instead. I recommend using Google. Using the Google search engine, if I want to search the "learn" section only -- and as an example I'll search for everything which mentions "Montage":
There are two components to this search:
1) The site is "yamahasynth.com/resources" (starts with "site:" which tells Google search to only look in the "Learn" section. When I "hover over" the "Learn" section at yamahasynth - the browser tells me in the bottom-left corner (for Chrome) that the "Learn" uses the link "yamahasynth.com/resources".
Note: I usually search the entire site without restricting which area to search. And usually I'm looking for a forum post - still I use the entire site. For this, you would modify the "site:" string to "site:yamahasynth.com"
2) The second half is the search string or strings (I have "montage" - you can have more strings to further narrow down the search).
When viewing messages in each category, the first post, on a line above the date/time, shows the post's category. For example, in this category - this message shows "By Site Feedback" and posts in the MOXF section would show "By MOXF". The word "By" followed by the category name.
Therefore, if you wanted to restrict your search to just the MOXF forum category (as an example) -- and say I was looking for posts that mention the word "memory", then I would use: