[DOCS] Add cross-link from FileSystemShell.md to FileSystem API

[Username46786]

What

Add a short “See also” paragraph pointing from the HDFS shell docs to the FileSystem API.

Why

Helps readers discover programmatic equivalents of common HDFS shell operations.

How

• Edited hadoop-project-dist/hadoop-*/src/site/markdown/.md to include the API cross-link.

Notes

Trivial docs-only change; no JIRA ticket required

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 57s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 1s		detect-secrets was not available.
+0 🆗	markdownlint	0m 1s		markdownlint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ trunk Compile Tests _
+1 💚	mvninstall	57m 58s		trunk passed
+1 💚	mvnsite	1m 28s		trunk passed
+1 💚	shadedclient	102m 17s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 1s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 23s		the patch passed
+1 💚	shadedclient	42m 18s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 39s		The patch does not generate ASF License warnings.
		148m 57s

Subsystem	Report/Notes
Docker	ClientAPI=1.51 ServerAPI=1.51 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7975/1/artifact/out/Dockerfile
GITHUB PR	#7975
Optional Tests	dupname asflicense mvnsite codespell detsecrets markdownlint
uname	Linux 880895e5e8f5 5.15.0-143-generic #153-Ubuntu SMP Fri Jun 13 19:10:45 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 091bd8b
Max. process+thread count	529 (vs. ulimit of 5500)
modules	C: hadoop-common-project/hadoop-common U: hadoop-common-project/hadoop-common
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7975/1/console
versions	git=2.25.1 maven=3.6.3
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