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GH-39291: [Docs] Remove the "Show source" links from doc pages (#40167)



### Rationale for this change
To fix the show source button links to 404 page problem

### What changes are included in this PR?
The show source button link will be removed.

### Are these changes tested?
Not yet

### Are there any user-facing changes?
Yes

* Closes: #39291
* GitHub Issue: #39291

Authored-by: Divyansh200102 <divyanshkhatri200102@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>