What are the main sections of a good project documentation?

The question posted is unclear.
If you mean what documents are typically produced as part of a project then these tend to vary from non saftey system to safety criticials system.
They are also dependent on the disciplines, so for software deliverables the assurance would vary to that of a Civils construction.
Below is list of documentation that you could be expected to deliver and again is dependent on the scopes of works along with the company assurance process etc: Design Management Plan (DMP) Systems Engineering Management Plan (SEMP) Requirements Management Plan Human Factors Integration Plan (HFIP) Human Factors Task Analysis Reliability, Availability & Maintainability (RAM) Plan Reliability Availability & Maintainability (RAM) Status Report Interface Management Plan EMC Control Plan Verification & Validation Plan Verification & Validation Report Configuration Management Plan EMC Test Specification / Plan Obsolescence Strategy Engineering Safety Management Plan Architecture Description EMC Technical File Engineering Safety Case Engineering Safety & Assurance Case (ESAC) System Safety Case Power Loading Application Safe Load Assessment Track Clearance Approval Disability Discrimination Act Access Statement Agreed Interface Definition Document (AIDD) Engineering Safety Hazard Log Design Reviews Conceptual Design Statement (CDS) Concept Report Acceptance Certificate Design Check Certificates Compliance Report / Declaration System Capability Performance Report Congestion, Control & Emergency Plan (CCEP) Update & Engineering Plan Change To Rolling Stock (CRS) Fire Compliance Submission LFEPA Exemption Fire Safety Compliance Inspection Report LU FCT Authority To Use Notice Asset Performance Readiness Plan Asset Performance Readiness Report Operational Readiness Plan Operational Readiness Report Railway Migration Strategy & Plan Inspection & Testing Strategy / Plan Asset Register Verification Report Consent To Test / Trial Report Snagging Register Quality Inspection Completion Certificate (QICC) Certificate Of Technical Conformance (CTC) Staged Completion Report Completion & Consent To Operate Report /Engineering Completion Declaration Project Completion & Handover Certificate / Delivery Into Service (DIS) Mandatory Asset Information Deliverables (MAID) Consents Strategy & Plan Restrictions Register Report Change Safety Analysis (CSA) Report Train Service Operator Training (part of ORR) Route availability (part of ORR) Detailed train path slots (part of ORR) Ops trial test.(part of ORR) SUP FRACAS procedure Testing Readiness Statement Briefing Note (instructions to staff on train) Integration testing safety justification Integration testing readiness statement System Integration Test Specification System Integration Test Plan FRACAS Report Infrastructure controller letter GM Consent letter ATC Related Simulators Acceptance Certificate Rescue & Recovery Plan ( Passenger Hours) Emergency Response Unit (ERU) training (review) Train Maintenance Regime - All Fitted Trains Supplier Product Safety Case Agreed Maintenance Boundary Definition Document Maintenance Training Maintenance Manuals Site Specific Maintenance Readiness Report Works Test report Functional Test Reports Letter Of No Objection RAM Analysis Report RAM Case Reliability Growth Report Consent to Install Safety QRA Interim Maintenance Readiness Report Failure Modes Effects and Criticality Analysis (FMECA) Availability Modelling Report Environmental stress screening (ESS) test reports RAM Requirements Specification Maintainability Analysis Report Reliability test scripts Maintainability Demonstration Reports Reliability Growth Test Plan Sevice Controller Training Operator Manuals Initial Change Safety Analysis Report (iCSA) Life Cycle Cost (LCC) Report