- Add black, djhtml, prettier
- Add precommit


This PR add configurations for black, djhtml and prettier.

- black is used for formatting python files
- djhtml for formatting html files. Use `{# fmt: off #}` and `{# fmt: on
#}` where needed.
- prettier for formatting of sass,css,yml and js files

Note: prettier picks up some of it's config from `.editorconfig`, so
it's updated appropriately

Formating of mardown files, minified or vendored js files are ignored.

Formatting used off-the-self rules for better consistency with the
community as large.

The formatting is not applied to all the files as it will introduce a
lot of merge conflict with
current PRs, instead `pre-commit` config is provide which can be
installed by `precommit install`
locally. It will format only the modified files in the PR/commits. 

Once majority of the current active PRs are either rebased & formatted
using precommit or merged, we can
run the `make fmt` on the full repo and activate additional prettier and
black checks in our lint rules.

The modified sass files fixes the eslint error that would came up if we
were to apply the prettier formatter on them.

Fixes #3500

When following docs to set up a development environment, without this
change one would see this error:

> Unknown command: 'wagtailupdatesite'. Did you mean
> wagtail_update_index?

Fixes: #3112

---------

Co-authored-by: Saurabh Kumar <theskumar@users.noreply.github.com>

Run Django and docs preview server, and watch and compile frontend
changes in parallel.

This lets you quickly spin up and run everything in a single terminal.

Fixes #3242

Fixes: #3250

- Add page last edited info on each page
- Update header - site title, repo name & icon
- Disable custom fonts, use system-fonts
- Add copyright info in footer
- Update navigation structure and behaviour
- Add source code view and edit buttons
- Refractor documentation