Simplify the version bump process. ### Description

Simplify the version bump process.

- Use substitution hook in docs to update CDN URL.
  - In `dev` or `prerelease` versions, it will point to `dev` url
  - In `stable` versions, it will point it `vX.Y.Z` url
- Introduce a version bump script `tools/bump_version.py`.


~- Use `bump2version` to bump Python versions (continuing from #2460)~
~- Use `npm version` to bump JS versions~
~- Update `run_docker` script to use latest deployed pre-built docker image when `--pre-built` option is specified.~

### Checklists

- [x] Add new / update outdated documentation

package:
  name: attrs
  version: 21.4.0
source:
  sha256: 2d27e3784d7a565d36ab851fe94887c5eccd6a463168875832a1be79c82828b4
  url: https://files.pythonhosted.org/packages/be/be/7abce643bfdf8ca01c48afa2ddf8308c2308b0c3b239a44e57d020afa0ef/attrs-21.4.0-py2.py3-none-any.whl
requirements:
  run:
    - six
test:
  imports:
    - attr

package:
  name: attrs
  version: 21.4.0
source:
  sha256: 2d27e3784d7a565d36ab851fe94887c5eccd6a463168875832a1be79c82828b4
  url: https://files.pythonhosted.org/packages/be/be/7abce643bfdf8ca01c48afa2ddf8308c2308b0c3b239a44e57d020afa0ef/attrs-21.4.0-py2.py3-none-any.whl
requirements:
  run:
    - six
test:
  imports:
    - attr
