Added FerretDB2/PostgreSQL Docs.

Thanks to juri_ at WeKan Libera.Chat IRC and xet7 !

docs/Platforms/FOSS/FerretDB2-PostgreSQL.md

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+# Install WeKan, FerretDB 2, PostgreSQL
+
+## WeKan
+
+```
+git clone --branch main --depth 1 https://github.com/wekan/wekan.git
+cd wekan
+sudo apt update
+sudo apt install -y build-essential gcc g++ make git curl wget \
+p7zip-full zip unzip unp npm p7zip-full
+sudo npm install -g n
+export N_NODE_MIRROR=https://github.com/wekan/node-v14-esm/releases/download
+sudo -E n 14.21.4
+sudo npm -g install @mapbox/node-pre-gyp
+sudo npm -g install meteor@2.14 --unsafe-perm
+export PATH=$PATH:/home/demo/.meteor
+meteor npm install production
+meteor build .build --directory --platforms=web.browser
+```
+
+## Postgres 17 + DocumentDB
+
+```
+sudo bash -c 'curl -fsSL https://repo.pigsty.io/pig | bash'
+pig repo add all -u
+pig ext install pg17
+pig ext install documentdb
+```
+Edit `/etc/postgresql/17/main/postgresql.conf`, there set
+```
+shared_preload_libraries = 'pg_cron, pg_documentdb, pg_documentdb_core'
+```
+Restart PostgreSQL:
+```
+sudo service postgresql restart
+```
+
+## FerretDB
+
+Download:
+```
+curl -L  \
+https://github.com/FerretDB/FerretDB/releases/download/v2.7.0/ferretdb-amd64-linux.deb \
+-o /tmp/ferretdb-amd64-linux.deb
+```
+Install:
+```
+sudo apt -y install /tmp/ferretdb-amd64-linux.deb
+```
+Edit your `/etc/systemd/system/ferritdb.service` file,
+add your username/password pair to the following line:
+```
+Environment="FERRETDB_POSTGRESQL_URL=postgres://ferret:DB_PASSWORD_GOES_HERE@127.0.0.1:5432/postgres"
+```
+Then enable and start FerretDB:
+```
+sudo systemctl enable ferretdb.service
+sudo service ferretdb start
+```
+
+## Initializing the Database
+```
+su -
+su - postgres
+CREATE ROLE ferret WITH PASSWORD 'DB_PASSWORD_GOES_HERE';
+CREATE DATABASE ferretdb;
+GRANT ALL PRIVILEGES ON DATABASE ferretdb TO ferret;
+ALTER ROLE ferret WITH LOGIN;
+CREATE EXTENSION documentdb CASCADE;
+GRANT USAGE ON SCHEMA documentdb_api to ferret;
+GRANT USAGE ON SCHEMA documentdb_core to ferret;
+GRANT USAGE ON SCHEMA documentdb_api_internal to ferret;
+GRANT USAGE ON SCHEMA documentdb_api_catalog to ferret;
+GRANT INSERT ON TABLE documentdb_api_catalog.collections to ferret;
+GRANT ALL ON SCHEMA documentdb_data to ferret;
+GRANT documentdb_admin_role to ferret;
+```
+## Launching WeKan
+```
+export PATH=$PATH:/home/demo/.meteor
+cd ~/wekan
+
+MONGO_URL=mongodb://ferret:DB_PASSWORD_GOES_HERE@127.0.0.1:27017/wekan \
+WRITABLE_PATH=.. WITH_API=true RICHER_CARD_COMMENT_EDITOR=false \
+ROOT_URL=https://wekan.example.com meteor run \
+--exclude-archs web.browser.legacy,web.cordova \
+--port 8080 2>&1 | tee ../wekan-log.`date +%s`.txt
+```
+
+## Notes
+
+- Machine must have at least 3 gigs of ram. Crashes at npm installing meteor, with 384 megs.
+- Machine must have at least 4 gigs of ram. Crashes at meteor build with 3 gigs.
+- Be ready to read rebuild-wekan.sh if you want to actually get it running.