add ppl proposal docs for future ppl language improvements

Signed-off-by: YANGDB <[email protected]>

.github/ISSUE_TEMPLATE/ppl_command_request.md

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+---
+name: 🎆 PPL Command request
+about: Request a new PPL command Or syntax change
+title: '[PPL-Lang]'
+labels: 'enhancement, untriaged'
+assignees: ''
+---
+**Is your feature request related to a problem?**
+A clear and concise description of what the PPL command/syntax change is about, why is it needed, e.g. _I'm always frustrated when [...]_
+
+**What solution would you like?**
+A clear and concise description of what you want to happen.
+ - Add Example new / updated syntax
+ - [Optional] Add suggested [ANTLR](https://www.antlr.org/) suggested grammar  
+
+**Add Proposal Document**
+
+Under the [docs/planning](../../docs/ppl-lang/planning) folder add a dedicated page for your suggested command or syntax change
+
+_**Example Proposal Document**_
+
+See [ppl-fillnull-command.md](../../docs/ppl-lang/planning/ppl-fillnull-command.md) example
+
+**Do you have any additional context?**
+Add any other context or screenshots about the feature request here.

OpenSearch-PPL-Command-Process.md

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+# OpenSearch PPL Command Development Process
+This document outlines the formal process for proposing and implementing new PPL commands or syntax changes in OpenSearch.
+## Phase 1: Proposal
+### 1.1 Create GitHub Issue
+
+Start by creating a new GitHub issue using the following [template](.github/ISSUE_TEMPLATE/ppl_command_request.md):
+```
+name: PPL Command request
+about: Request a new PPL command Or syntax change
+title: '[PPL-Lang]'
+labels: 'enhancement, untriaged'
+assignees: ''
+---
+
+**Is your feature request related to a problem?**
+A clear and concise description of what the PPL command/syntax change is about, why is it needed, e.g. _I'm always frustrated when [...]_
+
+**What solution would you like?**
+A clear and concise description of what you want to happen.
+- Add Example new / updated syntax
+- [Optional] Add suggested [ANTLR](https://www.antlr.org/) suggested grammar
+
+**Add Proposal Document**
+Under the [docs/planning](../../docs/ppl-lang/planning) folder add a dedicated page for your suggested command or syntax change
+
+See [ppl-fillnull-command.md](../../docs/ppl-lang/planning/ppl-fillnull-command.md) example
+
+**Do you have any additional context?**
+Add any other context or screenshots about the feature request here.
+```
+
+### 1.2 Create Planning Document PR
+Create a Pull Request that adds a new markdown file under the `docs/ppl-lang/planning folder`. This document should include:
+
+1) Command overview and motivation
+2) Detailed syntax specification 
+3) Example usage scenarios 
+4) Implementation considerations 
+5) Potential limitations or edge cases
+
+### Phase 2: Review and Approval
+
+1) Community members and maintainers review the proposal 
+2) Feedback is incorporated into the planning document / PR comments
+3) Proposal is either accepted, rejected, or sent back for revision
+
+### Phase 3: Experimental Implementation
+Once approved, the command enters the experimental phase:
+
+1) Create implementation PR with:
+    - Code changes
+    - Comprehensive test suite
+    - Documentation updates
+
+2) Code is clearly marked as experimental using appropriate annotations 
+3) Documentation indicates experimental status 
+4) Experimental features are disabled by default in production
+
+### Phase 4: Maturation
+During the experimental phase:
+
+1) Gather user feedback 
+2) Address issues and edge cases 
+3) Refine implementation and documentation 
+4) Regular review of usage and stability
+
+### Phase 5: Formal Integration
+When the command has matured:
+
+1) Create PR to remove experimental status
+2) Update all documentation to reflect stable status
+3) Ensure backward compatibility
+4) Merge into main PPL command set
+
+### Best Practices
+
+* Follow existing PPL command patterns and conventions
+* Ensure comprehensive test coverage
+* Provide clear, detailed documentation with examples
+* Consider performance implications
+* Maintain backward compatibility when possible
+
+### Timeline Expectations
+
+* Proposal Review: 1-2 weeks
+* Experimental Phase: 1-3 months
+* Maturation to Formal Integration: Based on community feedback and stabilityOpenSearch PPL Command Development Process