Fix Docker volume mount PWD usage across integration tutorials

## Problem

Several integration tutorial files use `$(PWD)` in Docker volume mount commands, which attempts command substitution but fails because PWD is an environment variable, not a command. This causes Docker volume mounts to fail with empty paths.

## Current Issue

In the Linux/macOS sections, the pattern `--volume=$(PWD):/demo` tries to execute a command named 'PWD' which doesn't exist, resulting in:
- `PWD: command not found` error
- Empty volume mount path
- Failure to access files like `@/demo/init.sql`

## Files Affected

Based on codebase analysis, the following file currently has this issue:
- `docs/integrate/oracle/tutorial.md` (Line 45)

Interestingly, the same file correctly uses `--volume=${PWD}:/demo` in the Windows PowerShell section (Line 53), showing the correct syntax.

## Other Files Reviewed

The following files use correct syntax with `$PWD` (environment variable):
- `docs/integrate/influxdb/tutorial.md` (Line 50): `--volume="$PWD/var/lib/cratedb:/data"`
- `docs/feature/search/fts/analyzer.md` (Lines 169-170): `--volume="$PWD/synonyms-*.txt:/crate/config/"`

## Solutions

Replace `$(PWD)` with either:
1. `$PWD` - Use the environment variable directly
2. `$(pwd)` - Use command substitution with lowercase `pwd` command

## Technical Context

- `$(PWD)` = Command substitution attempting to run 'PWD' command (fails)
- `$PWD` = Environment variable containing current working directory (correct)
- `$(pwd)` = Command substitution running 'pwd' command (also correct)

## Related

- PR: https://github.com/crate/cratedb-guide/pull/261
- Original discussion: https://github.com/crate/cratedb-guide/pull/261#discussion_r2280885928

This issue was identified during review of the Oracle integration tutorial and should be fixed to ensure proper Docker volume mounting functionality.