HDDS-14776. Developer Guide for ZDU

[errose28]

What changes were proposed in this pull request?

• Add a new Develop section to the developer guide to cover developer documentation about writing code.
  • This caused a large amount of renames with no content changes
• Add a developer guide for handling upgrade compatibility with ZDU under this new Develop section.

Note that pnpm build will fail until the user doc link is committed, and the design doc linked here is also not yet committed. The PR will remain as draft until both of those are merged.

What is the link to the Apache Jira?

HDDS-14776

How was this patch tested?

Local preview of the page using pnpm start.

[dombizita]

Thanks for working on this @errose28! Few nits and small comments inline, also asked Claude to check for missed out ideas, these two make sense:

• No general protobuf-compatibility principle up front. Several sections repeat "additive optional fields are safe / unknown field vs unknown method." A short shared paragraph in Background (unknown field is tolerated, unknown method/enum-required-semantics is not) would reduce repetition and give a single reference point.
• Config / ozone-site.xml compatibility during mixed versions isn't mentioned. New config keys or changed defaults that alter persisted/wire behavior mid-upgrade are a real compatibility surface — worth at least a sentence, even if only to declare it out of scope.

I don't insist to add these, just wanted to highlight them.

[errose28]

Thanks for the review @dombizita. All comments should be resolved now. A general protobuf section and config handling section are good additions as well. Let me put some thought into it and update the doc accordingly.