docs: restore eight markdown links mangled into NUL bytes

[devGregA]

Description

Five documentation files contained literal NUL bytes (0x00) that broke eight inline markdown links. In every case a placeholder of the form \0<index>\0 sits exactly where the link destination ](url) belongs, so the link renders as raw text:

File	NULs	Broken links
connectors/upstream/toolreference.md	6	Semgrep token page, Snyk REST base, Snyk EU REST base
get_started/about/about_defectdojo.md	4	Open-Source demo, Pro demo
get_started/contributing/how-to-write-a-parser.md	2	acunetix.md example page
get_started/contributing/documentation.md	2	http://localhost:1313
issue_tracking/pro_integration/messaging_connectors.md	2	https://api.slack.com/apps

The NUL bytes had a second effect beyond the eight broken links, and it is the reason this went unnoticed: grep reports each of these files as Binary file ... matches and suppresses the matching lines, so the broken text never shows up in a search. For example, grep -rn "Snyk REST API" returned only Binary file ... matches rather than the line. git diff is affected for three of the five — it renders them as Binary files differ — while toolreference.md and how-to-write-a-parser.md still diff as text because their first NUL byte falls past git's 8000-byte binary-detection window.

Origin and how the destinations were recovered

All eight placeholders trace to a single commit, 1cafb09 (#15661). The pattern is consistent with a link-protecting pass that swaps each destination for a NUL-delimited sentinel, rewrites the surrounding prose, and restores the destinations afterwards — with the restore step dropping these eight. Indices are per-file counters (17, 18, 19, 22, 77, 389, 394, 395).

Nothing here is guessed. For each file, the last revision before #15661 still holds the intact text; it was located by walking that file's history and counting NUL bytes per revision until one came back clean. Every repaired line is byte-identical to its pre-#15661 counterpart, and all eight restored links turn out to be self-links (display text equals destination), which is a useful independent sanity check.

toolreference.md has a second corroborating source: its four translated siblings (.de, .es, .fr, .ja) were never touched by #15661 and carry the same markup. They agree with the parent revision on all three destinations. Those four files need no change — 0 NUL bytes each, links intact.

Scope note

One pre-existing inconsistency was deliberately left alone, because it predates the corruption and appears identically in all five language files: the Snyk link text reads /rest while the destination points at /v1. The /rest value is the one the docs instruct you to enter in the Location field, so only the decorative href is stale. Worth a follow-up, but changing it here would mean an unexplained edit sitting on top of an otherwise byte-exact restore.

Test results

• python -c "d=open('docs/content/connectors/upstream/toolreference.md','rb').read(); print(d.count(b'\0'))" → 0
• Repo-wide scan: 0 text files under docs/ contain NUL bytes (images excluded).
• Regex sweep across all of docs/content for markdown link openers whose next two characters are not ]( → 0 hits, so the defect is fully enumerated rather than partially patched.
• Every repaired line verified byte-identical to its pre-change all docs references from Products to Assets #15661 revision.
• No CR bytes introduced; files stay LF-only per .gitattributes (*.md text eol=lf).
• grep now reads all five files as text.
• Docs rendering is covered by the existing validate_docs_build workflow; no local Hugo toolchain was used.

Documentation

This PR is the documentation change.

[devGregA]

GitHub cannot render a text diff for three of the five files: the old blob has a NUL byte inside git's 8000-byte binary-detection window, so they show as Bin / +0/-0 above. Those changes are reproduced here so they can be reviewed without fighting the binary diff.

Each NUL byte is written as <NUL> below. In every case the only edit is the placeholder becoming the link destination - nothing else on the line changed.

docs/content/get_started/about/about_defectdojo.md

@@ line 108 @@
-A running example of DefectDojo (Open-Source Edition) is available at [https://demo.defectdojo.org/<NUL>18<NUL>.
+A running example of DefectDojo (Open-Source Edition) is available at [https://demo.defectdojo.org/](https://demo.defectdojo.org/).
@@ line 113 @@
-[https://pro.demo.defectdojo.com/<NUL>19<NUL>.
+[https://pro.demo.defectdojo.com/](https://pro.demo.defectdojo.com/).

docs/content/get_started/contributing/documentation.md

@@ line 21 @@
-4. Visit [http://localhost:1313<NUL>17<NUL>.
+4. Visit [http://localhost:1313](http://localhost:1313).

docs/content/issue_tracking/pro_integration/messaging_connectors.md

@@ line 38 @@
-1. Go to [https://api.slack.com/apps<NUL>22<NUL> and select **Create New App**, then **From scratch**.
+1. Go to [https://api.slack.com/apps](https://api.slack.com/apps) and select **Create New App**, then **From scratch**.

The other two files (connectors/upstream/toolreference.md, get_started/contributing/how-to-write-a-parser.md) diff as text in the Files tab, because their first NUL byte falls past that 8000-byte window.