doc: add age(1) and age-keygen(1) man pages

Closes #131

.github/workflows/ronn.yml

@@ -0,0 +1,22 @@
+on:
+  push:
+    paths:
+      - '**.ronn'
+name: Generate man pages
+jobs:
+  ronn:
+    runs-on: ubuntu-latest
+    name: Ronn
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Run ronn
+        uses: ./.github/workflows/ronn
+        id: ronn
+      - name: Commit and push if changed
+        run: |-
+          git config user.name "GitHub Actions"
+          git config user.email "actions@users.noreply.github.com"
+          git add -A
+          git commit -m "doc: regenerate groff and html man pages" || exit 0
+          git push

.github/workflows/ronn/Dockerfile

@@ -0,0 +1,8 @@
+FROM ruby:3.0.1-buster
+
+RUN apt-get update && apt-get install -y groff
+RUN bundle config --global frozen 1
+COPY Gemfile Gemfile.lock ./
+RUN bundle install
+ENTRYPOINT ["bash", "-O", "globstar", "-c", \
+    "/usr/local/bundle/bin/ronn **/*.ronn"]

.github/workflows/ronn/Gemfile

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gem "ronn", "~> 0.7.3"

.github/workflows/ronn/Gemfile.lock

@@ -0,0 +1,20 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    hpricot (0.8.6)
+    mustache (1.1.1)
+    rdiscount (2.2.0.2)
+    ronn (0.7.3)
+      hpricot (>= 0.8.2)
+      mustache (>= 0.7.0)
+      rdiscount (>= 1.5.8)
+
+PLATFORMS
+  aarch64-linux
+  x86_64-linux
+
+DEPENDENCIES
+  ronn (~> 0.7.3)
+
+BUNDLED WITH
+   2.2.15

.github/workflows/ronn/action.yml

@@ -0,0 +1,4 @@
+name: Ronn
+runs:
+  using: docker
+  image: Dockerfile