Index the topics

[infotexture]

[stale]

This issue has been automatically marked as stale because it has not been updated recently. It will be closed soon if no further activity occurs. Thank you for your contributions.

[lief-erickson]

Working on it...

[infotexture]

Index entries were merged to source files for 3.3.3. The PDF index will be enabled for 3.4 with #264.