[FEATURE] Add --incremental and --include-current flags for faster local docs builds

[sankalps0549]

Is your feature request related to a problem? Please describe.
Building documentation with sphinx-multiversion rebuilds every tagged version from scratch on each run. For a repository with many version tags, this is slow and wasteful during local development. Additionally, there is no way to preview documentation for the current working tree (e.g. an in-progress release) in the multi-version docs output — you must create a git tag first, which is not practical during development.

Describe the solution you'd like
Two new flags for run.py build-docs:

• --incremental — Only build documentation for version tags that don't already have an output directory. Previously built versions are preserved, significantly speeding up repeated local builds.
• --include-current — Read version.json and, if it contains a version newer than the latest git tag, build documentation for the current working tree and include it in the version switcher dropdown. This allows previewing unreleased documentation without creating a tag.

API Design (if applicable)

# Full rebuild (existing behavior, unchanged)
python run.py build-docs

# Incremental: only build tags that are missing from the output
python run.py build-docs --incremental

# Incremental + include current working tree version
python run.py build-docs --incremental --include-current

# Also available on the standalone switcher command
python run.py generate-switcher --include-current

Additional context

• In production/CI, the default full build is used — --incremental and --include-current are development conveniences only.
• --incremental requires a clean working tree (enforced via git status --porcelain) since it checks out tags in-place.
• version.json is always validated against the latest git tag to catch version progression errors (e.g. skipping v27.0.1 by jumping from v27.0.0 to v27.0.3) regardless of whether --include-current is set. The flag only controls whether the version appears in the built output.
• After an incremental build, the generated switcher.json is distributed to all version output directories so every version's dropdown stays in sync.