feat(docs): generate builtin.txt (#24493)

- eval.lua is now the source of truth.

- Formatting is much more consistent.

- Fixed Lua type generation for polymorphic functions (get(), etc).

- Removed "Overview" section from builtin.txt
  - Can generate this if we really want it.

- Moved functions from sign.txt and testing.txt into builtin.txt.

- Removed the *timer* *timers* tags since libuv timers via vim.uv should be preferred.

- Removed the temp-file-name tag from tempname()

- Moved lueval() from lua.txt to builtin.txt.

* Fix indent

* fixup!

* fixup! fixup!

* fixup! better tag formatting

* fixup: revert changes no longer needed

* fixup! CI

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Co-authored-by: zeertzjq <zeertzjq@outlook.com>

.gitattributes

@@ -3,6 +3,7 @@
 *CMakeLists.txt linguist-language=CMake
 
 runtime/doc/*   linguist-documentation
+runtime/doc/builtin.txt linguist-generated
 
 runtime/lua/vim/_meta/vimfn.lua linguist-generated