Improve & document NPM package addition process

At this moment addition of the new NPM package to Lexical monorepo requires repetitive updates to ~10 different files. On top of that Meta specific WWW build requires separate changes done to internal codebase in order to keep it running.

**Example:**
In #5861 we reverted #5830 that added `@lexical/devtools-core` package. Later it was re-added in #5864 with fixes done to the build system.

**Scope:**
- Document steps necessary to add new NPM package to Lexical monorepo. Specifically:
- PR description format to highlight new package addition
- Changes to the shared files necessary
- Merge procedure
- Reduce number of changes needed to add new package