docs: fix existing-checkout import and low-memory build instructions

[yeseorizi]

Problem

The installation guide first clones POSIM, then imports a manifest containing that existing checkout. With Ubuntu's vcstool 0.3.x, --shallow --skip-existing raises AttributeError: 'NoneType' object has no attribute 'startswith' on existing Git repositories.

Also, colcon --executor sequential serializes packages but does not restrict compiler jobs within each package. A 4 GiB ARM64 build can still be killed for lack of memory.

Changes

• Remove --shallow from the import command in both English and Korean guides; retain --skip-existing.
• Add the same optional low-memory command to both guides, using MAKEFLAGS="-j1" and CMAKE_BUILD_PARALLEL_LEVEL=1. The former also prevents colcon's Makefile backend from adding its own parallel job count.
• No installer, CI, Docker tag or release-status changes.

Validation

• Used the current repository manifest with three existing Git checkouts and matching origin URLs: original import failed; revised import exited 0 and preserved all three checked-out HEADs.
• A sensor-plugin build killed under the initial parallel configuration completed with the documented job limits. Both sensor and ROS plugin packages built under those limits.
• Node 24.19.0 / pnpm 10.11.0: generated 50 EN/KO pages and passed the standalone website check (3,321 local links/anchors).
• Reviewed the changed EN/KO installation sections at desktop and 390 px viewport widths; no page-level horizontal overflow. Code blocks remain independently scrollable.

This is a focused installation-instruction correction, not a claim that every supported platform has passed a new clean installation.