Organize README commands and update descriptions

[google-labs-jules]

This PR organizes the README.md file by moving the web server instructions into the main CLI commands section (web Group). It also updates the pd db downgrade description to warn that it is destructive and adds documentation for the pd db populate command.

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PR created automatically by Jules for task 16662085729627837264 started by @hunsche

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[github-actions]

Code Coverage

File	Stmts	Miss	Cover	Missing
source/public_detective/cli
analysis.py	208	1	98%	260, 267–>239, 276–>239, 282–>239
config.py	56	2	96%	9, 36
db.py	88	1	97%	89–>94, 143
progress.py	20	0	100%
web.py	11	0	100%
worker.py	33	0	100%
source/public_detective/exceptions
analysis.py	2	0	100%
source/public_detective/models
analyses.py	112	0	99%	171–>176
candidates.py	30	0	100%
file_records.py	56	3	95%	31, 58, 69
procurement_analysis_status.py	15	0	100%
procurements.py	138	0	100%
source_documents.py	20	0	100%
source/public_detective/providers
ai.py	143	0	95%	121–>126, 129–>154, 136–>145, 139–>137, 142–>137, 146–>149, 165–>170, 274–>280, 316–>314
config.py	70	0	97%	114–>117, 117–>120
config_manager.py	19	0	100%
database.py	55	12	76%	40–42, 53–>111, 60–80, 123–125
date.py	2	0	100%
file_type.py	80	0	100%
gcs.py	29	0	100%
http.py	42	0	100%
image_converter.py	26	0	100%
logging.py	43	0	98%	83–>93
office_converter.py	36	0	100%
pubsub.py	76	0	98%	96–>99, 114–>117
secrets.py	10	0	100%
source/public_detective/repositories
analyses.py	167	4	97%	935–936, 1013–1014
budget_ledgers.py	22	0	100%
file_records.py	38	0	98%	66–>69
procurements.py	379	2	99%	569, 665–>664, 667–>664, 695
source_documents.py	30	0	100%
status_histories.py	25	0	100%
source/public_detective/services
analysis.py	755	17	97%	286–287, 434, 572–575, 593–>600, 1135–>1128, 1384–1388, 1394–1395, 1398–1399, 1588
converter.py	57	2	97%	147, 191
pricing.py	55	1	96%	71
ranking.py	98	1	98%	149
source/public_detective/web
main.py	16	1	94%	32
pages.py	27	0	100%
presentation.py	100	0	96%	173–>176, 176–>193, 179–>190, 190–>177, 245–>244
source/public_detective/worker
subscription.py	145	2	98%	242–>245, 275, 295
TOTAL	3334	49	98%

Tests	Skipped	Failures	Errors	Time
643	0 💤	0 ❌	0 🔥	39.522s ⏱️