Move off Trac - Execution - landing page

[btlogy]

Following #4095 (Move off Trac - Initiation) and in parallel with #4161 (Move off Trac - Execution - Forgejo), we can now start executing the migration of the landing page as a part of the plan.

In short, the goal is to switch the landing page of ​https://tahoe-lafs.org/ from the Trac Wiki system hosted on the Linode server to the Jekyll static page on the new webforge server hosted by Hetzner.

This while using proxy/rewrite rules to keep serving the other pages (tickets, wiki, downloads) from the Linode server (waiting for the next migration phase).

Most of the following steps could have been reviewed and automated with PRs, but will have to be manually executed for the lack of better DNS delegation (see #4162) and the current broken CI (see #4182):

0. verify the landing page and the rewrite rules
1. set the TTL of the A record of tahoe-lafs.org to 600 sec
2. wait at least for the previous TTL to expire (10800 sec?)
3. update the A record of tahoe-lafs.org from 74.207.252.227 to 135.181.155.146
4. add an AAAA record for tahoe-lafs.org with 2a01:4f9:c011:b882::1
5. reconfigure the nixosConfiguration of webforge to handle tahoe-lafs.org (virtualhost + certificate)
6. verify the landing page and the rewrite rules

Notes:

• we can not use CNAME instead of A/AAAA records because they would conflict with other records (MX/TXT)
• the downtime will be longer than necessary because the above steps will have to be executed asynchronously (and manually) by different people (e.g. meejah and btlogy/ben).