Docs/schema change

[PGijsbers]

Change

Adds documentation on how to change the schema when adding an attribute to an existing entity.

How to Test

Make sure you install the documentation dependencies (pip install -e ".[docs]" or refer to pyproject.toml).
Read and follow the documentation, see if it works, makes sense, and is intuitive.

Checklist

• Tests have been added or updated to reflect the changes, or their absence is explicitly explained.
• Documentation has been added or updated to reflect the changes, or their absence is explicitly explained.
• A self-review has been conducted checking:
  • No unintended changes have been committed.
  • The changes in isolation seem reasonable.
  • Anything that may be odd or unintuitive is provided with a GitHub comment explaining it (but consider if this should not be a code comment or in the documentation instead).
• All CI checks pass before pinging a reviewer, or provide an explanation if they do not.

Related Issues

See comment below.

[PGijsbers]

Some images for pages like the README currently do not load. This is because with the current organisation they could not render from the docs/readme.md page at the same time as from the docs/developer/index.md page. This would either require a separate post-processing plugin or to restructure the documentation. I will do this in a follow up PR. The documentation is currently not published anyway.

[Taniya-Das]

The documentation instructions are clear to me.

[PGijsbers]

@Taniya-Das Can I have an review looking at:

• The new readme (docs landing page, docs/readme.md)
• The new contributor guide (docs/contributing.md)
• The trimmed old dev guide (docs/developers/index.md).. it's still in progress but i moved some stuff out and pics should work
• Whether you think the current level of work-in-progress is acceptable for a merge.
  Reason that I would want to merge is that I don't have the time to finish the rewrite this week, and others might be more inclined/find it easier to help this way.

Other material wasn't changed (much) since last review. Thank you very much!