Provide extension list in overview and add make target (#1102)

I reckon the docs overview should have some short description of the extensions. Not entirely sure on the correct terminology usage of `twtxt.net`, `yarnd` and `yarn.social`.

Co-authored-by: Lysander Trischler <twtxt@lyse.isobeef.org>
Reviewed-on: #1102
Reviewed-by: James Mills <james@mills.io>
Co-authored-by: lyse <lyse@noreply@mills.io>
Co-committed-by: lyse <lyse@noreply@mills.io>

Makefile

@@ -1,5 +1,5 @@
 -include environ.inc
-.PHONY: deps dev build install image release test clean tr tr-merge
+.PHONY: deps dev build install image release test clean tr tr-merge docs
 
 export CGO_ENABLED=0
 VERSION=$(shell git describe --abbrev=0 --tags 2>/dev/null || echo "$VERSION")
@@ -96,3 +96,6 @@ tr:
 
 tr-merge:
 	@goi18n merge -outdir ./internal/langs ./internal/langs/active.*.toml ./internal/langs/translate.*.toml
+
+docs:
+	@cd docs; jekyll s

docs/index.md

@@ -4,3 +4,42 @@ title: "Developer Documentation for twtxt (the project)"
 ---
 
 # twtxt - Developer Documentation
+
+## Twtxt Extensions
+
+There are some optional and fully backwards-compatible extensions to the
+original
+[Twtxt File Format](https://twtxt.readthedocs.io/en/latest/user/twtxtfile.html#format-specification)
+and
+[Twtxt Discoverability](https://twtxt.readthedocs.io/en/latest/user/discoverability.html)
+Specifications.
+
+All those extensions have the goal to improve interaction and some of them are
+the result of what a lot of Twtxt feeds had settled on anyways, long before
+yarnd was born. So not all of them are inventions by
+[twtxt.net](https://twtxt.net/), but rather just a formalization of what was
+already being actively used by the Twtxt community.
+
+None of these extensions are required, they're completely optional, but of
+course we do recommend them. If you decide to build (some of) them into your
+client, this ranking might help you getting started with the most important
+ones. Of course, any ranking is up to debate, but we do believe that the first
+two are the most useful ones clients should have.
+
+1. [Twt Hash Extension](/doc/twthashextension.html)
+   in combination with
+   [Twt Subject Extension](/doc/twtsubjectextension.html)
+   for forming conversation trees
+2. [Multiline Extension](/doc/multilineextension.html)
+   for writing longer twts with paragraphs
+3. [User-Agent Extension](/doc/useragentextension.html)
+   for discovering feeds with multi-user clients
+4. [Metadata Extension](/doc/metadataextension.html)
+   for additional feed metadata like description
+5. [Archive Feeds Extension](/doc/archivefeedsextension.html)
+   for archiving and rotating long feeds
+
+## Yarnd Documentation
+
+1. [API Documentation](/doc/api.html)
+2. [Technology Stack](/doc/techstack.html)