use make html from the main Makefile to build guides

[evgeni]

What changes are you introducing?

Use make html with the main Makefile instead of calling make html BUILD=… in the guides/ subdirectory.
The result is the same, but that way we rely on the html target to know all builds instead of having to define them manually

Why are you introducing these changes? (Explanation, links to references, issues, etc.)

To avoid things like #4564

Anything else to add? (Considerations, potential downsides, alternative solutions you have explored, etc.)

Contributor checklists

• I am okay with my commits getting squashed when you merge this PR.
• I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• Foreman 3.17/Katello 4.19
• Foreman 3.16/Katello 4.18 (Satellite 6.18; orcharhino 7.6)
• Foreman 3.15/Katello 4.17
• Foreman 3.14/Katello 4.16 (Satellite 6.17; orcharhino 7.4; orcharhino 7.5)
• Foreman 3.13/Katello 4.15 (EL9 only)
• Foreman 3.12/Katello 4.14 (Satellite 6.16; orcharhino 7.2 on EL9 only; orcharhino 7.3)
• We do not accept PRs for Foreman older than 3.12.

[github-actions]

The PR preview for 8f8495b is available at theforeman-foreman-documentation-preview-pr-4567.surge.sh

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