Add 'attribute' content type

[aneta-petrova]

What changes are you introducing?

Adding the attribute content type to all attribute files.

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

asciidoctor-dita-vale relies on the attribute to recognize attribute files

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

N/A

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 bdc8667 is available at theforeman-foreman-documentation-preview-pr-4631.surge.sh

No diff compared to the current base

show diff

[aneta-petrova]

Ready for review.

No diff compared to the current base

This tells me adding this particular content attribute doesn't affect anything.

[maximiliankolb]

LGTM

[aneta-petrova]

Merged to "master" and cherry-picked:

111f011..d624eea 3.18 -> 3.18
f87e34c..15d0fd7 3.17 -> 3.17 (minor conflict, due to some attr files not being present)
b68d3c4..6984157 3.16 -> 3.16