Added docs about using MongoDB at unsupported CPUs with Qemu.

Thanks to xet7 !

Related #4321,
related #5814

docs/Databases/mongodb-avx-qemu.md

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+## Meteor podcast about using MongoDB at unsupported CPUs
+
+- https://www.youtube.com/watch?v=bnU9bUVeN04
+- Making MongoDB working with Qemu: https://github.com/wekan/wekan/issues/4321#issuecomment-3006557279
+
+## MongoDB Operating System Support precompiled binaries
+
+- arm64: Only Ubuntu. No Raspberry Pi OS, Alpine Linux.
+  - But installing .deb packages to Raspberry Pi 5 running Raspberry OS 64bit can have hardware running cooler, than on Ubuntu.
+- For anything else, see MongoDB download page.
+
+## MongoDB CPU support
+
+New MongoDB supports newer CPUs only, like:
+- x86_64 that have AVX instructions, from MongoDB 5 and later
+- ARMv8.2-A microarchitecture, from MongoDB 4.4.19, 5.0, 6.0 and later, like:
+  - Raspberry Pi 5
+  - OrangePi 5
+  - Apple Silicon arm64
+
+Old MongoDB 4.4.18 supports old CPUs, like:
+-  Intel Core 2 Duo, CPU does not support AVX instructions
+-  ARMv8.0 microarchitecture, like Cortex A53/A55/A72:
+  - Raspberry Pi 3, Cortex-A53, https://en.wikipedia.org/wiki/Raspberry_Pi
+  - Raspberry Pi 4, Cortex-A72
+  - Orange Pi 3, https://en.wikipedia.org/wiki/Orange_Pi
+
+Info about requiring newer arm64:
+- https://www.mongodb.com/community/forums/t/mongodb-community-6-0-5-illegal-instruction-core-dumped-ubuntu-18-04-on-cortex-a72-aarch64/223970/3
+
+Detecting does x86_64 CPU support AVX:
+- https://github.com/wekan/wekan/issues/4321#issuecomment-2469332492
+
+Error running MongoDB 8 at RasPi4, when not using Qemu:
+
+```
+December 06 11:48:49 rpi4b systemd[1]: Started mongod.service - MongoDB Database Server.
+December 06 11:48:53 rpi4b mongod[3749]: /usr/bin/mongod: line 4:  3750 Illegal instruction     (core dumped) /usr/bin/mongodreal --co>
+December 06 11:48:53 rpi4b systemd[1]: mongod.service: Main process exited, code=exited, status=132/n/a
+December 06 11:48:53 rpi4b systemd[1]: mongod.service: Failed with result 'exit-code'.
+```
+## a) Prebuilt binary
+
+MongoDB 7.3.4 for RasPi4 and older:
+- https://github.com/123swk123/mongodb-armv8-a/releases/tag/v7.3.4-alpha
+
+## b) Compile MongoDB, takes a lot of time
+
+CrossCompiling MongoDB from x86_64 to ARMv8.0 Cortex A53/A55/A72 like RasPi4 and older:
+- https://github.com/123swk123/mongodb-armv8-a
+
+Compiling MongoDB from x86_64 to x86_64 CPUs that does not have AVX instructions:
+- https://github.com/GermanAizek/mongodb-without-avx/blob/main/Dockerfile
+
+## c) Run MongoDB with Qemu, that supports newest CPU features
+
+qemu-user can run single Linux executeable for many architectures. It does not emulate full OS like qemu-system.
+
+Running MongoDB with Qemu on x86_64 CPU that does not support AVX, like Intel Core 2 Duo:
+- https://github.com/stevekerrison/mongo-qemu-avx
+
+Running MongoDB with Qemu on older arm64, like RasPi4 and older RasPi:
+- https://github.com/xet7/simpletasks/blob/main/install-mongodb.md

docs/Databases/mongodb-raspi4-qemu.md

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+## Meteor podcast about using MongoDB at unsupported CPUs 
+
+- https://www.youtube.com/watch?v=bnU9bUVeN04
+- Making MongoDB working with Qemu: https://github.com/wekan/wekan/issues/4321#issuecomment-3006557279
+
+## Installing MongoDB 
+
+MongoDB 8 arm64 works at M1 Air arm64 macOS and Linux.
+
+But because MongoDB 8 core dumps at RasPi4, running it here with Qemu.
+
+1. Install Ubuntu 24.04 arm64 for Raspberry Pi
+
+2. Install MongoDB 8 repo for Ubuntu 24.04 arm64
+
+3. Install deps:
+
+```
+sudo apt -y install qemu-user
+```
+
+4. Rename MongoDB to different filename:
+
+```
+sudo mv /usr/bin/mongod /usr/bin/mongodreal
+```
+
+5. Edit start scipt for Qemu MongoDB
+
+```
+sudo nano /usr/bin/mongod
+```
+
+6. Copy paste start script for Qemu MongoDB
+
+It uses qemu-user to run MongoDB,
+and passes all command line arguments to MongoDB.
+
+```
+#!/bin/bash
+/usr/bin/qemu-arm64 /usr/bin/mongodreal --config /etc/mongod.conf
+```
+
+7. Save and exit nano: Ctrl-o Enter Ctrl-x Enter
+
+8. Make script executeable
+
+```
+sudo chmod a+x /usr/bin/mongod
+```
+
+9. Start and run MongoDB
+
+```
+sudo systemctl enable mongod
+
+sudo systemctl start mongod
+```
+