Doc: Add Core Internals handbook under docs/ - #1971

Summary

Adds a developer-oriented handbook for the Cloudberry kernel under
docs/core-internals/ — 21 chapters, 104 sections, organised into five
parts after the structure of Rogov's PostgreSQL 14 Internals.

Coverage: storage and data layout (heap, AO/AOCS, PAX, indexes, the
catalog), transactions and MVCC (isolation, snapshots, vacuum, locks and
the global deadlock detector, distributed transactions and 2PC), query
processing (stages, statistics, the PostgreSQL planner, distributed
planning, ORCA, the executor, Motion and the interconnect), high
availability and recovery, and resource management.

Read the handbook as a PDF (652 pages, 20 MB) so you can
review the content without building the site. All 113 diagrams are rendered
as vector SVG and the text is selectable and searchable. It was produced by
printing the handbook's HTML edition from a headless browser, so layout and
typography match the source. The file is a release asset, deliberately not
part of this diff — a 20 MB binary does not belong in the repository's
history.

What the chapters contain

Why here, and how the site uses it

The directory is written for Docusaurus so that apache/cloudberry-site
can pull it in as a git submodule with a sparse checkout limited to
docs/core-internals, served by its own docs plugin instance with an
autogenerated sidebar — the same arrangement already used for the PXF
documentation (apache/cloudberry-site#372). Keeping the source next to the
code it describes means a change to the engine and the change to its
description can travel in one commit.

A follow-up PR against cloudberry-site will add the submodule and the
three config touch points. It has to come second, since the submodule
needs a commit on this side to point at.

Nothing here depends on site-level configuration except one small MDX
component: 37 figures are CSS-laid-out HTML rather than SVG, and are handed
to that component as a string so MDX does not try to parse them as JSX. The
component and its CSS are a few lines and are described in the README.

Verification

• Docusaurus 3 / MDX v3, built locally: no MDX compilation errors, no
  broken links, no broken anchors, 23 pages, all 64 figures resolved.
• Apache RAT: every one of the 92 files carries the ASF license header,
  so no pom.xml exclusions are needed.
• No environment-specific detail: host names, addresses and install
  paths from the machine the sessions were captured on are replaced with
  documentation-safe equivalents of identical width, so the column
  alignment of captured output is preserved. Verified across the markdown,
  the HTML and the PDF.

Testing

• Unit tests added/updated
• Integration tests added/updated
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance: None. No code is touched.

User-facing changes: New documentation. Nothing changes for existing
users until cloudberry-site links it in.

Dependencies: None added to this repository. A site that renders the
diagrams needs @docusaurus/theme-mermaid; without it the mermaid blocks
degrade to code blocks and everything else renders unchanged.

Checklist

• Followed contribution guide
• Added/updated documentation
• Reviewed code for security implications
• This PR contains AI-assisted code generation
• Requested review from cloudberry committers

Additional Context

On maintenance. The chapters describe a specific point in the tree's
history. Prose is the durable part; line numbers are the perishable part.
The title="file:line" convention is machine-readable on purpose — a CI
job can verify that every cited path still exists, which catches the worst
class of drift (a file that moved or was deleted) cheaply. I would rather
agree on that check in review than add it unilaterally here.

On the size of the review. This is a large diff and I do not expect
anyone to read 29k lines in a PR. Reading order that costs the least: the
attached PDF for the content, docs/core-internals/README.md for the
layout and conventions, and one chapter of your choice for the texture —
ch02 is the shortest, ch18 the most demanding. Chapter-level corrections
are welcome as ordinary follow-up PRs.

On the branch base. This branch is cut from an April snapshot of main
rather than today's tip, so GitHub will report it as behind. That is an
artifact of how it was pushed, not of the content: all 93 files are new
paths under docs/core-internals/, nothing else is touched, and the diff
GitHub computes is exactly those 93 additions. Happy to rebase onto current
main if you would rather review it that way.

On AI assistance. Disclosed above per AI_GUIDELINE.md. The generation
was AI-assisted; the technical claims, the code citations and the captured
sessions are mine to answer for, and I will answer review questions
directly rather than through a tool.