docs: centralize documentation into helpdoc (#15)

* docs: centralize documentation into helpdoc

Problem: documentation was spread across four files (README.md,
doc/oil.txt, doc/api.md, doc/recipes.md) with duplication and gaps.
User events were undocumented, and adapter docs only lived in the
README.

Solution: expand doc/oil.txt with five new sections (introduction,
requirements, adapters, recipes, events) from existing content. Trim
README to a landing page pointing to :help oil. Delete doc/api.md
and doc/recipes.md since their content now lives in the helpdoc.

Closes: barrettruth/oil.nvim#6

* build: remove docgen pipeline

Problem: the Python docgen pipeline (scripts/generate.py,
scripts/main.py, nvim_doc_tools) was designed for upstream's doc
layout and is incompatible with the centralized helpdoc structure.
It overwrites doc/oil.txt entirely and expects sections in README.md
that no longer exist.

Solution: delete the pipeline scripts (generate.py, main.py,
requirements.txt), remove the update_docs CI job, and clean up the
Makefile and .gitignore references. Linting and typechecking remain
unchanged.

.github/workflows/tests.yml

@@ -70,24 +70,6 @@ jobs:
         run: |
           bash ./run_tests.sh
 
-  update_docs:
-    name: Update docs
-    runs-on: ubuntu-22.04
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Install Neovim and dependencies
-        run: |
-          bash ./.github/workflows/install_nvim.sh
-
-      - name: Update docs
-        run: |
-          python -m pip install pyparsing==3.0.9
-          make doc
-      - name: Check docs are up to date
-        run: |
-          git diff --exit-code README.md doc
-
   release:
     name: release
 
@@ -97,7 +79,6 @@ jobs:
       - stylua
       - typecheck
       - run_tests
-      - update_docs
     runs-on: ubuntu-22.04
     steps:
       - uses: googleapis/release-please-action@v4