docs: add docmentation on template API stability

[lnielsen]

Stable APIs for templates is criticial for backward compatibility. This includes the

• Context
• Blocks
• Macros

Following should be considered breaking changes:

• Modification and removals in the template context (additions are ok).
• Additions, modification and removals of template blocks.

Other code could be impacted as well, such as Inline HTML/CSS classes (for instance for WTForms

[AtharvMixraw]

Hello! @lnielsen
I'm interested in contributing to improving the documentation about bundles and compatibility.

Is this issue still relevant and open for contributions? I have experience with Python, documentation, and would love to help improve the Invenio documentation.

If this is still needed, could you provide any guidance on:

• Specific areas of documentation that need improvement
• Preferred documentation format/style
• Any existing work or discussions on this topic

I'm eager to contribute to the Invenio project and would appreciate any direction you can provide.

Thank you!