Add ability to generate TOC file for testing links

[ShimShtein]

• [X] I am familiar with the contributing guidelines.

This commit adds the ability to generate a TOC file that can be used to test documentation links. This is done in the theme PR: https://github.com/RedHatSatellite/foreman_theme_satellite/pull/31

Please cherry-pick my commits into:

• [X] Foreman 3.9/Katello 4.11 (planned Satellite 6.15)
• [ ] Foreman 3.8/Katello 4.10
• [ ] Foreman 3.7/Katello 4.9 (Satellite 6.14)
• [ ] Foreman 3.6/Katello 4.8
• [ ] Foreman 3.5/Katello 4.7 (Satellite 6.13; orcharhino 6.6)
• [ ] Foreman 3.4/Katello 4.6 (EL8 only)
• [ ] Foreman 3.3/Katello 4.5 on EL7 & EL8 (Satellite 6.12 on EL8 only; orcharhino 6.4/6.5 on EL8 only)
• [ ] Foreman 3.2/Katello 4.4 on EL7 & EL8
• [ ] Foreman 3.1/Katello 4.3 on EL7 & EL8 (Satellite 6.11 EL7/8; orcharhino 6.3 on EL7/8)
• We do not accept PRs for Foreman older than 3.1.