docs(qdp): drop dead ADDING_INPUT_FORMATS.md links from readers README

[bearomorphism]

What

Slice (A) from #1285 (per the status comment). qdp/docs/readers/README.md has two markdown links to ../ADDING_INPUT_FORMATS.md — a file that has never existed in the repo (see history below). lychee.toml carried an explicit exclude with a TODO: fix the links in this file note because of these.

Why

• The link checker had to ignore the entire file to keep CI green, hiding any future link rot in this README.
• Readers landing on the page see a 404-shaped reference for the "extension guide".
• The published doc on the same topic (docs/qdp/readers.md) had already chosen the right pattern for this — a plain-text ADDING_INPUT_FORMATS.md (TODO) marker — so the in-source README was the inconsistent one.

History — did ADDING_INPUT_FORMATS.md ever exist?

No. The placeholder was committed without the target file ever being written, and was then propagated forward by routine docs work. Verified:

• git log --all --full-history --diff-filter=AD -- "**/ADDING_INPUT_FORMATS*" → 0 commits: no commit ever added or deleted such a file.
• git log --all -S "ADDING_INPUT_FORMATS" --reverse shows where the string entered the repo and how it spread:

Date	Commit	Author	What
2025-12-30	9780d955a (PR #753)	@ryankert01	Introduced qdp/docs/readers/README.md with the broken [../ADDING_INPUT_FORMATS.md](../ADDING_INPUT_FORMATS.md) references and a ADDING_INPUT_FORMATS.md # Extension guide entry in the file-organization tree. First and only origin of the string.
2026-01-26	b736eb90d (PR #914)	@ryankert01	"[Docs] init docs effort" copied the same readers content into docs/qdp/readers.md, propagating the placeholder into the published site.
2026-02-04	dee2170bc	@guan404ming	"Deploy new Docusaurus website" — site build artifact (asf-site branch).
2026-02-11	930c0a3fe (PR #1040)	@suyashparmar11	"Update the docs versioning" — froze the placeholder into version-0.4 / version-0.5 snapshots.

So the link was a forward-looking pointer to an extension guide that the original author intended to write but never did. Two and a half releases later, it's still a TODO. Rather than block this PR on writing the guide, this fix matches what the published page already does — keep the placeholder visible as plain text — and unblocks the link checker.

How

• qdp/docs/readers/README.md — replace the two dead [../ADDING_INPUT_FORMATS.md](../ADDING_INPUT_FORMATS.md) markdown links with `ADDING_INPUT_FORMATS.md` (TODO) plain text, matching docs/qdp/readers.md lines 142 and 226. The placeholder remains visible to anyone who wants to write the extension guide later, but is no longer a broken link.
• lychee.toml — drop ./qdp/docs/readers/README.md from exclude_path (and its TODO: fix the links in this file comment) so CI''s link checker covers the file again.

Out of scope (left to @ryankert01 or follow-ups):

• Writing the actual ADDING_INPUT_FORMATS.md content.
• Reconciling docs/qdp/readers.md ↔ qdp/docs/readers/README.md duplication / "File Organization" stale tree.
• Other [Docs] Enhance docs #1285 sub-tasks (migration docs, more examples).

Verification

Reproduced and verified locally with lychee 0.24.2.

Before:

$ lychee qdp/docs/readers/README.md
[ERROR] file:///…/qdp/docs/ADDING_INPUT_FORMATS.md (at 137:5) | File not found
[ERROR] file:///…/qdp/docs/ADDING_INPUT_FORMATS.md (at 221:25) | File not found
🔍 2 Total ✅ 0 OK 🚫 2 Errors

After (with project config so the new exclude_path is honoured):

$ lychee --config lychee.toml qdp/docs/readers/README.md
🔍 0 Total ✅ 0 OK 🚫 0 Errors

(0 total because the only links in this file were the broken ones; the rewritten markers are now plain text. Nothing else in the README points outward.)

Pre-commit (end-of-file-fixer, trailing-whitespace) passes on the changed files.

Refs #1285

[bearomorphism]

checking the git history about ADDING_INPUT_FORMATS.md

[bearomorphism]

ok it looks just a placeholder for me

[ryankert01]

Not sure of this file ever existed before, but it(ADDING_INPUT_FORMATS.md) looks like should be in issue list not in file.