docs: bring back ALTSVC.md and HSTS.md

This partly reverts 0e06603b23

These file formats are not properly documented elsewhere, plus the
website uses these files to populate the documentation pages to which
users end up via the URLs that are mentioned within the alt-svc and hsts
files.

Fixes #15705
Reported-by: Jeffrey Bosboom
Closes #15706

docs/ALTSVC.md

@@ -0,0 +1,43 @@
+<!--
+Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+
+SPDX-License-Identifier: curl
+-->
+
+# Alt-Svc
+
+curl features support for the Alt-Svc: HTTP header.
+
+## Enable Alt-Svc in build
+
+`./configure --enable-alt-svc`
+
+(enabled by default since 7.73.0)
+
+## Standard
+
+[RFC 7838](https://datatracker.ietf.org/doc/html/rfc7838)
+
+# Alt-Svc cache file format
+
+This is a text based file with one line per entry and each line consists of nine
+space separated fields.
+
+## Example
+
+    h2 quic.tech 8443 h3-22 quic.tech 8443 "20190808 06:18:37" 0 0
+
+## Fields
+
+1. The ALPN id for the source origin
+2. The hostname for the source origin
+3. The port number for the source origin
+4. The ALPN id for the destination host
+5. The hostname for the destination host
+6. The port number for the destination host
+7. The expiration date and time of this entry within double quotes. The date format is "YYYYMMDD HH:MM:SS" and the time zone is GMT.
+8. Boolean (1 or 0) if "persist" was set for this entry
+9. Integer priority value (not currently used)
+
+If the hostname is an IPv6 numerical address, it is stored with brackets such
+as `[::1]`.