feat: Allow packages to define custom doc URL and badge

Co-authored-by: Jacob Oaks <jacoboaks.8@gmail.com>
Co-authored-by: r-hang <rhang@uber.com>

* Support dark mode theme

* Add dark mode to 404 and package pages

* Fix tests

Adds a `-templates` flag that can be used to provide
an alternative directory with templates for Sally to use.
The new templates override the default set embedded in Sally.

This includes a new 404 template so that 404 errors
use the same theme as the rest of the website.

Additionally, for HTTP status >400,
this also sets the Cache-Control header
to discourage CDNs like Cloudflare from caching the page.

Resolves #125, #18 

Adds support to other VCS systems as per the specification.
Defaults to `git` for backwards compatibility.
Renames the internally used `.GitURL` to `.RepoURL` for clarity.

Fixes the documentation URL in the homepage, as it already includes the protocol:

https://github.com/uber-go/sally/blob/686fb8782cfa89a807ecc6dcbc104c6dab43eb66/handler.go#L45

Right now, we're rendering "https://https://[...]".

<details>
<summary>Background</summary>

Sally renders two kinds of pages:

- packages: These are for packages defined in sally.yaml
  and any route under the package path.
- indexes: These list available packages.

The latter--indexes was previously only supported at '/', the root page.

This leads to a slight UX issue:
if you have a package with a / in its name (e.g. net/metrics):

- example.com/net/metrics gives you the package page
- example.com/ lists net/metrics
- However, example.com/net fails with a 404

</details>

This adds support for index pages on all parents of package pages.
Therefore, if example.com/net/metrics exists,
example.com/net will list all packages defined under that path.

Resolves #31

* update: removing redundant go-source tag

* fix: removing /.idea and adding dir to gitignore

* fix: removing /.idea and removing branch field from from templateData

* fix: pkgHndler defaultBranch rem revert

* fix: minor typo

* fix: removing assertion of template responses

* fix: tabs

* Update handler_test.go

* Update handler_test.go

* Update handler_test.go

---------

Co-authored-by: Anuragkillswitch <70265851+Anuragkillswitch@users.noreply.github.com>

Fixes up the `make lint` target to also check
that all files are properly gofmt-ed,
replaces the deprecated `golint` check with `revive`,
and sets up dependabot to also update tools and actions regularly.

Similar to the previous patch,
this patch removes knowledge of the configuration from the index handler.
Instead, the index handler is now passed a list of packageInfo structs
each containing just the information needed to render the index page.

This also obviates the need to duplicate the logic to compute
the package import path in the index.html template.

* template: Use a more fluid layout

Instead of using a table, take advantage of the grid layout.
We still print a table of sorts,
but it's more fluid in appearance based on width of the screen.

On narrower screens, we'll show a listing
where each item has a description label next to it
rather than at the top.

* Add support for package descrpitions

Packages may now optionally specify a description.
If specified, this is printed below the package information,
indented one column to make it stand out.

Co-authored-by: Sung Yoon Whang <sungyoonwhang@gmail.com>